Presumably not aircraft size in general
It covers all BA flying with 100 seats or more - thus A320 are within the agreement.
Is there a 'banned' destination list?
Yes - only problem is it appears to be a "state secret". It is contractually agreed between BA and BALPA and forms part of our contract of employment. Yes it does - I can see you all shaking your heads
The list is considered commercially sensitive and thus has never been released, but you can infer something about it by looking at the destinations served by GB and BMED. If MAN - DME isn't on it I will eat my (brand new) hat.......