From IEC 1010:
  • Cat I: Signal level, special eguipment or parts of equipment used in telecommunications or electronics.
  • Cat II: Local level mains, appliances and other portable equipment
  • Cat III: Distribution level mains, fixed installation wiring.
  • Cat IV: Service drop wiring to building(outside).

Personally, I'd never use a DMM for any thing closer to the Distribution wiring than the Main switchboard and I would most certainly not use one to test the incoming mains on the pole or at the metering.
IEC 1010 also does mention that DMM's used
for Electrical testing, must have the inputs marked with a certified Cat rating on them!. [Linked Image]