Dear,
When suppliers access your organization self registration link either from your corporate website or shared by you in an email. Then supplier faces the Ariba Self registration and he has to fill up the SPQ if set up. And you as the supplier approver get an email to review and approve the supplier org. When you follow the link, you reach the supplier workspace in SIPM of Ariba. This is the whole workflow. Please let me know if further details needed.
Regards
Sangram