When you're ready to start the payment approval process for a given project's invoices from a specific pay period, set the period as Ready to pay.
You must have the Accountant role at the company level in order to update a project pay period's status.
Setting a project's pay period as ready to pay automatically pushes all invoices for that project from that period to the payment approval step. You might do this after having received funding from the owner for the given project for this pay period.
To retract or prevent a given period's invoices from being eligible for review, set the pay period as Not ready to pay.