The temporary sponsor field is just for informational purposes only for reporting and reminders that the sponsor needs corrected within the first 90 days once the true sponsor joins.
Keep in mind, this is for when the associate names someone that has not yet joined. This is NOT to give them time to "think about" who their sponsor should be. All associates must declare their sponsor via the binding sponsorship agreement when they join.
If the temporary (substitute) sponsor decides not to join, the currently named sponsor will become their permanent sponsor. The temporary box will automatically uncheck once the 90 days have passed.