In which part of the database are the people who approve the purchase orders registered?

In which part of the database are the people who approve the purchase orders registered?
This is depends on the workflow editor setup, it could be a user or user group, the rules and condition you can find on this table.
You can try to find out from table below based on the relation on AOT (might be some other workflows table too depends the setup)
WORKFLOWWORKITEMTABLE
WORKFLOWASSIGNMENTTABLE
WORKFLOWELEMENTTABLE
WORKFLOWTABLE
WORKFLOWTRACKINGTABLE
WORKFLOWTRACKINGWORKITEM