How can we see the default value for PO SR Rule Name Prefix?

We have enabled auto-sourcing so when we create a BPA for a new supplier-item combination, a sourcing rule is automatically created, the name of which starts with the prefix PURCH_ and is concatenated with a system-generated number.  I would like to know where the default prefix is set up because I already checked the workflow attribute PO SR Rule Name Prefix but its default value is NULL.  Is there another place where this is defined?  Would appreciate any help.

Thanks,

Anna