This is not a terribly well-formed question, sorry. I don't really know what information is in the .aux file, or how I can make use of it in other (related) documents. What's the best reference for understanding how this works?
One common thing I would like to be able to do is the following: I have a book that contains lots of
\labels. I would like to be able to reference those labels from the solutions manual, which is separately created (as a .tex file) and generated (as a PDF).