I have searched in the application and noticed the following events availabile in Purchasing module.
1. These events are introduced for ASL integration with ERES. The ER documentation might have more info.
oracle.apps.po.asl.create -> PO ERES ASL Creation
oracle.apps.po.asl.update ->PO ERES ASL Update
oracle.apps.po.event.create_asl->Create New Approved Supplier List Entry
oracle.apps.po.event.update_asl ->Update Existing Approved Supplier List Entry
2. These events are launched during the XML Communication in PO Approval wf,after the PO is approved.
oracle.apps.po.event.setxml -> Set up PO via XML
oracle.apps.po.event.xmlpo -> Send PO via XML
oracle.apps.po.event.xmlpogen -> XML PO document is generated
oracle.apps.po.event.xmlposent -> PO Sent via XML
3. These events are newly introduced for Change Requests and signature flow (11.5.9 - 11.5.10.CU2)
oracle.apps.po.buyersignature -> Buyer Signature Event
oracle.apps.po.suppliersignature -> Supplier Signature Event
oracle.apps.po.event.requester_change -> Requester change Request pending
oracle.apps.po.event.suppchnresp -> Response to Supplier Change Request Event
oracle.apps.po.event.supplier_change -> Supplier Change, and PO is Responded
4. OTM Event - These event is launched for OTM integration flow, to send the PO details to OTM.
oracle.apps.po.event.document_action_event -> Document Action Event
5. Receiving Events
oracle.apps.po.group.receive -> PO ERES Receiving Group Event
oracle.apps.po.line.receive -> PO ERES Receiving Line Event
oracle.apps.po.rcv.deliver -> PO ERES Receiving Delivery
oracle.apps.po.rcv.inspect -> PO ERES Receiving Inspection
oracle.apps.po.rcv.rcvtxn -> Receive Transaction
oracle.apps.po.rcv.transfer -> PO ERES Receiving Transfer
Hope this helps.