Single Check for multiple assignment with Tax Multiple Payments as One= Yes

I have changed the below flags at Payroll level so that multiple assignments under on payroll are taxed together and employee gets one pay check per payroll.

multiple assignment: checked yes

Tax Multiple Payments as One= Yes

So the functionality is working. But my question is how the single payment is attached to the assignment. Is there any logic that it should be primary. What if the primary assignment has no earnings dirung the payroll run.

Lets says employee has 5 assignments during pay period and he gets one check. lets say each assignment has different personal payment method attached to that assignment. I would like to know how Oracle selects the payment method(assignment) to payout on single paycheck.