For most geometries, the choice of units is unambiguous. Standard MOPAC and Gaussian input is in Ångstroms, TURBOMOLE input is in atomic units. In these cases, do not use A0 or ANGSTROMS.
If A0 is present, the input geometry will be regarded as being in atomic units.
If ANGSTROMS is present, the input geometry will be regarded as being in Ångstroms.
If neither A0 nor ANGSTROMS are present, then the default will be used.