The regs mention met or perf reasons, which is obviously a pretty broad brush and will affect different operators/aircraft in different ways...
EU-OPS 1.295:-
(b) An operator must select and specify in the operational flight plan a take-off alternate aerodrome if it would not be possible
to return to the departure aerodrome for meteorological or performance reasons. The take-off alternate aerodrome,
in relation to the departure aerodrome, shall be located within:
1. for two-engined aeroplanes, either:
(i) one hour flight time at a one-engine-inoperative cruising speed according to the Aircraft Flight Manual (AFM)
in still air standard conditions based on the actual take-off mass; or
(ii) the operator’s approved ETOPS diversion time, subject to any MEL restriction, up to a maximum of two
hours, at the one-engine-inoperative cruising speed according to the AFM in still air standard conditions
based on the actual take-off mass for aeroplanes and crews authorised for ETOPS;