Not every building in Grennia uses this system, but most do.

The ground floor is Level 0. Go up and it becomes floor 1, 2, and so on.
The basement floors are labelled -1, -2, and so on.

Mezzanines are usually labelled Floor #.5 (eg: Floor 0.5) or Floor # 1/2 (eg: Floor 2 1/2), but plenty of buildings use M# (eg: M1, M2) for their scheme.
Basements can also be labelled B# (B1, B2) like in most other nations.

Below is a basic diagram of the Grennian floor numbering scheme.