Once an approval policy is set up, it's applied immediately to all unapproved payables matching the criteria. If the criteria aren't matched, then the payable(s) are automatically approved.
In practice, you can see next to each payable where the approval policy applies:
The different steps that apply to the payable (1)
How many approvals are required to complete a respective step (2)
The approvers within the different steps who can approve the payable (3)
Who and when someone gave an approval (or rejection) (4)
