(assuming you refer to Substance here, or similar) I think it's similar to code, where you should specify if you deliver the source code - which can be used to compile the program, or just the compiled program itself. If you just deliver the output, i.e. the compiled program, you can re-use the source since the (c) remains…
I have, and I assume most artists have as well, reused various effects, layers and overlays from psds created for one project on another. I would see reusing bits of substance designer files in the same light, and really can't see why it would be a problem. Maybe if the content in question is hugely important to the…