Data\menu
Data\menu\textures

contain files used in the menu interface
I am not sure if all of them are used


Data\menu\symbols\FE_buttons_48

contains the symbols used to select the faction in campaigns, custom battles and multiplayer
Each faction has 4 files:
symbol48_FACTION.tga
symbol48_FACTION_grey.tga
symbol48_FACTION_roll.tga
symbol48_FACTION_select.tga
The first one is a "static" button. The other three simulate the intermittence when selecting the faction.

Data\menu\symbols\FE_buttons_24

contains small symbols used in custom battles and multiplayer
Each faction has again 4 files:
symbol24_FACTION.tga
symbol24_FACTION_grey.tga
symbol24_FACTION_roll.tga
symbol24_FACTION_select.tga
with the same use as the buttons_48.

Data\menu\symbols\FE_flags

contains the flags used to represent the teams in custom battles and multiplayer.
I am not sure if they are used, as different colors are included but only red flags appear in game (to the best of my knowledge).