Bartoz, thanks a loot for your help. I am trying all what you propose in order to gain better understanding of abaqus.
Regarding the problem expossed, I have just solved it by
RP=a.ReferencePoint(point=( 1.0, 0.0, 0.0))
RP_id=RP.id
This give me the id of the ref point
then
ref_point=...