After the 20 May 2019 redefinition of units, this relationship is only nearly equivalent.
It also describes that no matter how complicated the formula is, we can quickly determine the Relative Molecular Mass of the compound by following some simple steps.
Since it is a ratio, it is unitless.
The concept of relative molecular mass can only be used for substances that are made up of molecules.
Prior to 1961 the unit amu atomic mass unit was used.
This article explains the concept of Relative Molecular Mass and its determination.
A similar calculation can be done for compounds.
Periodic Table and Isotopes The relative atomic masses of different elements on the periodic table is the average of the masses contributed from different isotopes.