The default for config in SRM is for Classic mode (i.e., Purchase Orders will be created in the backend system)
One of the standard settings for configuration is:
Supplier Relationship Management -> SRM Server -> Cross Applications Basic Settings -> Activate Extended Classic Scneario
Which causes SRM to generate a local purchase order for items in the shopping cart. Subsequent followon documents will be created locally and then replicated into R3.
I hope this clears things up a bit. There seems to be much confusion on this topic (Classic versus Extended Classic) which I have addressed here
in the Classic versus Extended classic decision matrix.
I hope this helps!
Best, Jim Stewart