I think the title speaks for itself:
What does "flat" MC samples mean?
Could you give some context?
I don't know how to put it in a context, I've heard things like "production of flat Monte carlos" etc...
Sounds like slang, and google hits are as good as non-existent.
Flat in some distribution (pileup?)?
Flat in terms of file format (no hierarchy of particles)?
No event weights?
Separate names with a comma.