We have different units for same physical quantity for our convenience.
Let us suppose we have to measure a distance between two atoms in a molecule we will probably work in Angstrom(Å) and in another scenario we have to calculate distance between stars so we will probably work in light year.
Therefore in both cases the physical quantity used is length but the units are different. So, the point is that the units can vary for different measurements but dimensions remains same for the physical quantity.
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