...The purpose of joining multiple mods into a collection is to make them share the source code. This allows joined work without the time consuming and bug prone process of moving modpacks between source codes.
This mean the source code is made up of multiple features, which can be tweaked by XML and as a result it is possible to make total conversion mods without knowing how to program.
What this is NOT is a bunch of nearly identical mods. The design goal is to make each mod so unique that the average player should not notice that the source code is shared.