What is the difference between between set and add

[TriPSs]

Describe the bug Not necessarily a bug but missing documentation?

What is the difference between Set${baseName}On${dtoNames.baseName}Input and Set${pluralBaseName}On${dtoNames.baseName}Input? Both seem to accept the same input attributes and output is also the same.

I also noticed that when I do disableUpdate = false I'm unable to add the relation to the record, is this correct behaviour?