Thanks everyone - it is now much clearer as you have xplained it. Interesting how some experienced users can explain something so much better than whoever writes the operator's manuals!!!
I have only been a member on this site for a matter or weeks (and a tractor owner for a matter of days) and it amazes me how much assistance everyone has given both in chosing the right machine for me and in helping in my first tentative steps in the seat.
Hopefully, after a few hundred hours in that seat I'll be able to help others in a similar sitaution to that I'm in now.
Graham