Modifications, or mods, are edits to the games Rez files. These installation files contain every aspect of the games visible features, such as 3d models (.LTB), and textures (.DTX). LTB files contain not only the models, but all the animations as well. Rez files are essentially a proprietary compression format used by the LithTech game engine. Through the use of a pack of software, anybody can easily extract these rez files and view all the files contained in them. They can also swap out files for other files, or edit them directly, and then 'inject' them back into the Rez file. Although against the terms of service, many players create these mods, and even more players download them to use. Mods can give an advantage to the user, but can also be only used to edit the colors of a gun or other textures. Mods that give an advantage are the ones that modify the 3d models of guns and other objects. For instance, a user can replace the M14's 3d model with the G36E's 3d model. When a user then equips and uses the M14, it will shoot as fast as the G36E, but have the same damage as the M14. One of the most famous modifications made for Combat Arms was the Super Kukhri, originally made by Connor of MPGH.net. There have been many modifications cloning this one, but it spearheaded the model changing era.
It is also possible to import models and textures from other games that use the LithTech engine such as 'Contract J.A.C.K.' and the 'No One Lives Forever' series of games.