ABI hash of that specific package build, which is needed because compiling with different optimization levels etc. will change what part of the internals gets exposed in the .hi file for inlining into other modules; mismatches there lead to *really* weird behavior. (If you're lucky, it'll "just" be a type mismatch in code you didn't write, because it came from the .hi file. If unlucky, it compiles but dumps core at runtime.)