What is the difference between cumulative dissertation and regular dissertation? The university is asking me whether or not my PhD thesis is a “cumulative dissertation”. I’m wondering what the difference would be for the university. By cumulative, do they mean stacking the published/submitted papers?
Cumulative dissertation is probably a literal translation of the German Kumulative Dissertation, which denotes a thesis by publication, compilation thesis or article thesis, i.e., a thesis which typically consists of some peer-reviewed publications, an introduction, and a conclusion.
The alternative to this is a monograph thesis, which is written separately as a coherent monolithic work and whose individual chapters are not intended for being read on their own.

The requirements on such a cumulative dissertation differ between universities and even faculties, so you have to look into their rules for those.