There are two pages that could be classed as 'Configuration'.

Account settings

The first, available only to logged-in users, allows changes to the information you provided when registering an account. This is reached via the 'Manage your account settings' link on the Tunes Front page, and discussed under the Accounts, logins, Registration, etc help page.

Display Settings

The second, discussed here, is the one reached via the 'Configure Display' link at the top left of every page, which allows considerable tweaking of what is displayed, and how. Many of these are available without being logged in; but if you are logged in there are (even) more, and you can save them, causing them to be used whenever you log in in future.

General Settings

A few little things :-

Default length for the text fields (in characters), Default height of listboxes (in lines)
Many of the controls can be configured individually (see Form Layout, below); these supply a value for any that don't have one given.
Open the editor in a different browser window
(Only available when logged in) This may depend on your browser settings. Allows you to edit a list of tunes with less shuttling between pages.

Tune list options

These control various aspects of the Tune list page(s).

Fields to display
By default, the fields listed for each tune are Title, Country of origin and Rhythm. Change this by selecting/deselcting your own choice of fields in the listbox. Title is not included in the list, it's always displayed. Selecting a large number of fields may have some impact on the performance of this site.
Sort Order
A drop-down list showing the fields to be displayed, from which you can select one to sort the list by. You can't sort by fields that aren't shown. If you choose a field other than Title (the default) they will then be sorted secondarily in title-order.
Private tunes
(Only available when logged in) Choose whether or not to include your private tunes in the listings
Include tunes with no value for sort-field
If included, tunes where this field has no value will be found at the end of the list
Renumber the X: field in ABC downloads
If selected, the X: fields in a downloaded ABC list will be assigned numbers starting from 1; otherwise, they won't be changed. Handy for those that want it, downright obscure to anyone else.
How many tunes per page, how much overlap ?
You can decide how many tunes you want to see per page, up to a hardwired limit of 300 tunes per page (see lists). "Overlap" is, how many tunes from the bottom of one page will appear at the top of the next.

Search / Filter options

Options for the Tune Search page. The 'simple' version of this is not configurable, but the 'fancy' page can be tweaked.

Field display style
The fields displayed are shown either as text fields, where you just type the values in from the keyboard, or as listboxes, where you can select multiple values from a list (usually - there are a few fields for which multiple values would be meaningless). Selecting a field in the "Show these fields as listboxes" listbox causes it to be displayed as ... well, as you'd expect. Fields not selected there will be displayed in the other way.
Form layout
See Form Layout, below

Tune Editor

As with the search form, the 'simple' form of the Tune Editor is not configurable, the 'fancy' form is.

Use fancy editor
You can toggle between the 2 forms of the editor page using the button on the page itself, but this provides a default setting. There should also be one of these for Search ??
ABC image-rendering program
Which of the external ABC programs will be used to turn your tune into a picture of the dots. You can specify this individually for each tune, this is used for any tunes where you haven't done that. See the notes on the ABC Programs for more on this.
Show these fields as listboxes
As for Tune Search, above
Show these fields
By default, the editor will only show a control for a field when there is a value for it. This will get one on the screen whenever the editor's called, even on a tune that doesn't have a value for it.
Never show these fields
This does the converse of the above (see the note on required fields).
Listboxes only show your own values for this field
(Only when logged in). Mormally, the listboxes show all values in use for a field, regardless of whose tunes provided them. Normally, I think, this is what's wanted, and what I want to encourage, for the sake of not having too many overlapping values in the Search page. But this will override that and hide all values except the ones you use yourself. I invented it to use with the Collection field, for which it's set as a default, because the site provides special, more convenient ways of using this than looking for it from the Search page. Though there's nothing to stop you doing this if you wanted to, of course. (It can also be handy for checking your tunes for variant values, etc).
Form layout
See Form Layout, below

Form Layout

This refers to the 'fancy' form, used to display individual header values in the search and editor pages. These are separately configurable, but the idea is the same in each (it is also used in the 'Bulk Edit' list-operations form, but this, for now, is not separately configurable, it uses the same settings as 'Search').

This form is made up of a number of individual 'controls' (each one either a text-field or a listbox, as above) pointing at individual data-fields. The "Form Layout" box here gives you a way of specifying which fields are offered and choosing their positioning and size. The forms also contain various buttons, but they're not configurable.

This box may contain one or more lines of text, indicating the fields you want shown, each field optionally followed by a number (all items should be separated by spaces). The fields can be indicated by their longer, more descriptive names, as used by the listboxes, or in the more concise ABC 'key' style. The numbers set the size of the field's associated control - the width of a text field (in characters), the height of a listbox. If it's a text field, the height is not configurable - it will be displayed as a single line, though most fields will actually accept multiple values, separated by a newline character, the 'Return' key. If it's a listbox, the width is decided by the browser and not controllable here. If a size isn't given, the default size will be used (this is itself configurable, see above).

In addition to the names of the fields, there are two other 'controls' that can be specified here, indicated by a pair of (fairly arbitrary) 'magic words :-

NEW
This stands for the 'New Value(s) and key(s)' widget, as discussed in the editor page; optionally followed by a number for its size as usual (this sets the width of the text field contained in the widget, it doesn't include the list-box at the left, whose height is set by the list-box default height). This can be used in both layout configurations
ABC
A text field showing the 'ABC body', the actual dots of the tune, optionally followed by a number for its width. This is only applicable to the editor, it will be silently ignored elsewhere. This field is always displayed in the editor form, it can't be hidden, being added after everything else if you don't specify it here. If the editor also needs to display other fields for a tune (those that have values but are not specified in the layout or configured to be 'always hidden'), they will also be appended, with the ABC body coming last.

Each line in this 'configure layout' box corresponds to a line across the form, with the fields you name displayed in that order from left to right. If the combined width of such a line is larger than the available screen, the 'overflowing' fields will 'wrap' down to fit, in the same way that text would. One outcome of this is that it's possible to specify all the fields you want on a single line, in which case re-sizing your browser window can cause them to migrate all over the screen in a moderately amusing manner. With care, and a certain amount of trial-and-error, it's possible to use this to achieve a fairly efficient use of the screen. I hate forms that require me to scroll up and down unnecessarily.

Others

Hidden ABC fields
This is only offered to logged-in users (of course). What it says. When other users look at your ABC, either on the View Tune page or as a download, any fields selected here will not be included. Use these fields to record things that wouldn't be meaningful to anyone else, or that you just plain want to keep to yourself for whatever reason. Not to be confused with the invent-your-own '%%' private fields with which it is too easily confused, especially by me. I need to sort this out
Bulk editing of a list of tunes, layout
Another header form layout just like the others.

the Buttons

When you have these values set as you wish ...