Great explanation BC, thanks. Right, so the Betfair website is set out with a line of prices, eg. (and I hope this formats okay).

Back Back Back Lay Lay Lay.

$4 $5 $6 $7 $8 $9.

$245 $576 $388 $122 $23 $15.

I back the horse at $6 and I want to have $100 on it but does that mean I can't because there's only $388 in the pool?

Similarly if I want to lay the horse at $7 does it mean I can only lay it for $122 divided by $7 (about $17) because if it wins, I have to pay out $122, or do I lay it for the full $122 knowing if the horse wins, I have to pay $122 x $7?