I'm not sure how much this clarifies things, and feel like it could be better. The comment "The signature-file file can be discarded afterward" implies that the signature-file is not useful after a delta has been calculated, but there are use-cases where keeping it is useful (like calculating another delta-file for a different new-file later).

A signature-file is a small binary signature of the old-file that can be used to generate a delta-file against a new-file. It is small compared to the whole old-file, so it can be easily transferred and/or stored to calculate a delta-file remotely and/or later against a new-file without access to the original old-file.

A delta-file is a small binary diff file that can be applied to the old-file to update it to the new-file. The delta-files contain only the differences between the old-file and the new-file, so they are generally much smaller than the whole new-file, making them efficient to transfer and/or store to regenerate the new-file remotely and/or later by patching the old-file.

I'll have a crack at writing something better.