This article needs to be completed, for now see here for more details
-mod is a command line function
Please note -mod:my_mod system works very differently for Rome Total War and Barbarian Invasion modifications.
For a RTW modification the -mod system is very user friendly with generally only changed files required in the mod folder, that mod folder can also be run under the BI.exe simply by adding -mod:my_mod to the shortcut for the BI.exe instead of the RTW.exe. The system works by looking first in the mod folder and then referring to the RTW/data folder for any files not present in the mod folder. The only files that must be present in the mod folder are those that give the map information, eg. data/text/regions_and_settlements_.... .txt, the contents of the data/world/maps/base and data/world/maps/campaign folders.
For a BI modification, however, almost the entire contents of the original BI/data folder have to be copied into the mod folder. This is because BI is already effectively a -mod of RTW and it is not possible under CA's set-up to have a multiple look-up that refers to the mod folder first, then BI/data, then RTW/data folder.
Even for BI modifications, the -mod:my_mod system is highly recommended, although it does not save as much space as for RTW mods, it does mean the users' original game installation is not affected by the modification, so other compatible mods can also be installed and the vanilla game still be played without the need for multiple installs.