An Atlas texture is a file with all the textures in it. For example the albedo. If you want to use trimsheets you just need to do the UVs over the image. Is compatible, you could do an Atlas of the trimsheets. In that image what is reusing is the geometry (albedo, normal and everything) of a wooden plank or a log. You can…
We have links to modular/trimsheet/atlas tutorials here, in case it helps to see more examples and walkthroughs http://wiki.polycount.com/wiki/Modular_environments