On my link to my main page on GIT I have a read me file giving a basic run down of what dats I change and what they are.
ROM/119/50.dat = is the title screen when you first log in and see the expansion pack titles listed to the right. The dat is just an alpha picture that was edited at one time to reflect what version I updated. I since quit doing that so that 50.dat file is just an untouched original title dat. I left it there so when people install mine they don't see a version number on that title screen anymore. Once I feel no one should be installing any older version of XI View I'll remove it. But again that 50.dat does nothing.
ROM/119/51.dat - I call the UI dat which is the US version for bazzar/linkshell icons, fonts, menu and macro colors changed to that almost black color ect.
ROM/91/15.dat is the JP UI dat, does same as above for JP.
ROM/280/15.dat is the menu icons, when you bring up your magic/abilities/weaponskill menu. It's those little icons in there. I have mine set up to use the old school icons which are just colors for each element instead of what SE changed to using graphical icons.
Aside from that I believe rom/287 dats 80-83 were some new ones found that were similar to the older 51 and 57 dats for UI and status icons but are not needed to replace at all.