Please check Section 3.5.2 of Multiwfn manual, the meaning of various options for defining the plotting plane has been described.
PS: Since dexamethasone is not a planar molecule, and there is no plane of great interest, plotting plane map may be not a good choice. Studying isosurface map of ELF/LOL instead may be more useful.
I wish to study localization of electrons in dexamethasone molecule. While executing ELF of the molecule in a plane, I saw lot of options such as XY plane 2: XZ plane 3: YZ plane 4: Define by three atoms etc. To plot the entire ELF of the molecule which option is suitable?