Our Ops manual is not so specific, I quote:
'If an APU located within the fuelling zone or which has an exhaust efflux discharging into the zone is stopped for any reason it should not be restarted until the flow of fuel has ceased and there is no risk of igniting fuel vapours'
I think the 757 APU is outside the zone and this is certainly the way most people in our company interpret it. In fact, with that in mind, I started the APU during fuelling the other day. Nobody said anything.
However, I have never seen the 'fuelling zone' defined.