Well, it's for the hold pool, so I'd guess they just fish a few pilots out of it as and when spaces appear on any fleet.
A330 fleet is gradually expanding, so that would seem the most likely.
A340 fleet is gradually being phased out, so that would seem unlikely.
747 fleet would depend on retirements, CUs and general attrition I guess.
I'm not in the know, just my $0.02