Atom fraction and mass fraction

Dear G4 team,

The G4Material class has two methods to access:
-The mass fraction with GetFractionVector()
-The number of atom fraction with GetAtomsVector()
I think I can use both these methods to get one of these quantities whatever the way I used to define my elements in the material (with a mass fraction or with a number of atoms).

However, the G4Element() has one method to access information on the isotopes. This method is GetRelativeAbundanceVector(). It is not clear to me if this method returns a mass ratio or a number of atom ratio. Could you please clarify?

In Geant4 “isotope abundance” has the same meaning as in literature : relative number of atoms of an isotope.
You can verify this with something like : G4cout << an G4Element;