Account derivation rules : how to default a value in a segment of a gl code combination ?

Hi ,

when  doing a payment I need that for gl bank account the value in a segment defaults from the the one defined in master supplier ( and does not pick up the entire code combinatation defined in bank accounts setup ) .

I think that Account derivation rules che do that but I am not able to do the setup ( documentation is not clear )

Anyone can help ? pls write at davide_miola@yahoo.it

regards