MFS sez:
Agree with the possibility that the worst case may not be SL; I think one of our types has a marginal max thrust increase with altitude for some temp conditions, due to the details of the flat rating applied.
A bit out of the ordinary, a custom rating for a particular operator at a particular airfield; probably the result of a negotiation by an OEM to forestall the operator buying a competitors' bird!
There are some economic (parts life) penalties associated with such a rating, but as long as it's with the certified limits ...
