If there are 'IN PROGRESS' transactions at the end of recoverying, what can the recoverying node do? If the 'IN PROGRESS' transaction is found, the recoverying RM can't determine it's going to be committed or aborted. The RM can't know even it is the end of transaction or not (it's still continuing).
So the recoverying RM must ask another node what to do for 'IN PROGRESS' transactions.
When asking, the recoverying RM must identify the transaction using the identifier of the originater node of the transaction and the transaction id on the originator node.
BTW, how does the recoverying RM know 'recoverying is completed'?