There's a severe lack of documentation on modding Borderlands 2. I find this to be a shame with the lively modding community Borderlands originally had. So, why not sponsor change? Do your part, sign up today!
Borderlands 2 packs with an obscure compression algorithm known as Lempel-Ziv-Oberhumer (LZO) . This lossless compression algorithm is renound for its speed. The game itself is based loosely on Unreal Engine 3. The Unreal Engine is just a collection of modern rendering techniques and design tools for primarily industrial use. That being said, the engine itself, and Unreal Script, is a dynamically typed language that compiles into a "cooked" distribution. That distribution is a packaging of game resources in a manner made for that platform such as the XBOX/PS3/PC. Unreal Bytecode is an abstraction layer that makes code you write once, and works on all intended platforms... That being said, one version of the game is just as complete as the others... The mechanisms that process and load the data are the only real differences. These mechanisms are often a hindrance: The save file for xbox360 must be within a certain limitation, this limitation puts a hard cap on BL2's weapon slots for that platform... Rather than write a different game for each platform this limitation is forced on the other platforms too. There is also shader limitations and other restrictions that limit the models and techniques allowed for those distributions. For the most complete game with the highest quality of "cooked" resources, the PC edition is tantamount.
- profile.bin - It is the account save file, it tracks global things such as golden keys, used shift codes, and the "community" bank claptrap so conveniently gives us.
- save####.sav - Willow Save Game file, contains character specific data. This is pretty publicly documented by Rick (aka gibbed) .
- <name>.upk - These files are default containers, part of the Unreal Engine . Upon deflation, these upk files are remarkably similar to other cooked UE3 games. As such, it is possible to create our own mods and to gleam game mechanics from this "bytecode" style engine.