There are a few definitions to what it means so undoubtedly you will get a few answer in here. My definition below. Dirty map: baked out ambient occlusion map used to mask ares of the model to make them look dirty in the cracks and corners. You can use this to dirty the diffuse texture if you have unwrapped the model in…
Fritz you can do either, if you composite you are limiting yourself to not tiling your diffuse texture and having extra work to mirror it. But yes that is what I did on my tank model as I stated above.