Paul,
Thanks again, I think I understand now.
I agree with you its a bit OTT to expect pilots to understand all of this.
Spitfire,
This is one of the more obscure bits of the ATPLs I could never understand and Paul is the first person I have come across who understands it. The rest of the stuff you need to learn is more obvious but it is detailed. The way I look at it is if you have a PPL and you want to pass the ATPLs then you probably will.
Good luck.