What does layer g mean?

The English name of building ground (i.e. 1 floor) is Ground floor, abbreviated as G.

UG is a floor above the first floor, which can be understood as the second floor or attic, or even as the compartment space between 1 floor and the second floor.

L 1 can be interpreted as the 3rd floor, and L2 can be interpreted as the 4th floor.

LG 1, that is, the low ground floor 1, is the underground floor, which can be understood as-1 floor or the basement floor.

LG2 is the second floor underground.