AryaWinla, the entries in the descr_model_battle.txt file have nothing to do with the unit cards, it simply tells the game what skeleton to use so the troops move correctly plus what sprites and textures are used to show the units on the battlefield, the former being 2D graphics shown at a distance, the latter being 3D skins for close up detail.

The link between unit and the unit card is simple a combination of the unit name and the faction. If you are playing the Julii and have created a unit called guys_with_big_swords, then you will need a TGA image file in Data\UI\Units\Romans_Julii\#guys_with_big_swords.tga (the # is required) to give you the little unit card you see in the build queue or in the army selection panel (on both strategy map and battlefield). If you select a unit to get more details - stats, unit description - you also get a larger picture, this is the TGA image file in Data\UI\Unit_Info\Romans_Julii\guys_with_big_swords_info.tga. And that's it, no need to edit any 'linking' file like descr_model_battle.txt.