Why are floors and ceilings not part of same composite?

[Christiaan]

I've noticed that the default composites slabs separate out floors and ceilings.

What's the idea behind this? Why not include them in the same composite?

[Erika Epstein]

Oops. now I see your AC version.
I don't know why, I am not a mindreader. Often it works better to model the ceiling separately from the floor.
-not all ceilings are as same height
-some areas have soffits
-separate ceiling leaves a space between it an the floor structure above where you can model HVAC etc.

[Karl Ottenstein]

...and also, the pre-defined composites in your country-specific default template come from your regional distributor. In the US, these predefined attributes are often not applicable to real work and are just examples.

For residential, I most often model the ceiling/floor assembly as a single composite. But, sometimes this is not desirable. Erika gave some examples. Another is where you use massing for preliminary design, and yet want to model floor structure (joists/beams) for construction documents. If you use a single composite, then if you reduce it to either ceiling gyp, or to subfloor/floor finish - then the rest has to be modeled again anyway. But, if the ceiling was modeled separate from the floor, then just the floor composite changes - and the structure, MEP, etc is inserted in the void during the design development phase (with the ceiling assembly perhaps shifting up/down based on final floor structure engineering).

Cheers,
Karl