What is the OOTB functionality for HSA FSA remaining pay periods.

Summary:

If emp changes his election on last day of the pay period, will that change be processed in that particular pay period run?

Take an example of the below pp. Suppose an employee is changing his HSA / FSA coverage as of 9/27/2026 which is the last day of the PP, will that chnage be processed in 20 2026 payroll run?

If not, I assume that the payroll would have been submitted 2 or 3 days before the PP end date. In such cases, how will we know which date to choose to identify the remining pay periods for right calculations?

Also, Do we suggest to enable Retro and arrears for HSA/FSA?