OK, I fired up BIAB and tried stuff, and this is what that display setting does:

If you say "dim = dim7", and you type dim5, it will be display as dim5 and dim7 will display as dim.

If you say "dim = dim5", and you type dim5, it will be display as dim and dim7 will display as dim7.

So if you have a mixture of these chords, toggling the setting will show either dims and dim7s, or dim5s and dims; you cannot see both dim5 and dim7 chords labelled as such, you have to choose.

If you prefer to see your four-note diminished chords labelled as dim7s, you have to say "dim = dim5" or BIAB will obnoxiously hide any 7s you type in. But then, if you just type "dim" you get a dim5.

(Seems to me they could have just let you use either dim or dim7 as synonyms for dim7, and required the 5 for dim5, but I guess fans of dim5s would object, so we get this wonderful flexibility.)

Last edited by Mark Hayes; 11/25/21 08:14 AM.