What is the difference between interior floor and interior ceiling?

I apologize for this stupid question.

In construction sets label, I have to assign a construction for interior floors and ceiling. I don't understand which is the difference.

in a multistory building, in a middle floor, the ceiling and the floor belong to the same construction and same stratigraphy.

In EnergyPlus (and OpenStudio) a surface belongs to a zone so the surfaces that separate one space/zone from another must be duplicated. If this wasn't the case one space/zone would have a ceiling but the one above it wouldn't have a floor. To illustrate this look at adjacent spaces in the SketchUp Plugin using Hide Rest of Model.

It's important to note that the ceiling construction of one space/zone must be a mirror image of the adjacent floor construction, same for walls, otherwise it will cause an E+ error (see image below for example).

Inheritance: Material > Construction > Building:Surface:Detailed > Zone

image description