Both mVA and MVA are correct unit but depends upon the
rating of transformer
1. mVA mean milli volt ampere, used for small rating
2. MVA means megga volt ampere used for large rating
MV.A (not 'MVA') is the symbol for megavolt ampere.and in some low volatge cases we consider VA that is (Voltage Ampere). where VA is the true vector sum of circuit power or reactive power.
So, i thik it is true. NOT 'mVA' in case of transformer.