Dates on Indexed Allocations

 I'm a bit confused with the various dates on Indexed Allocations.  My Indexed Allocation has a MO Recur Frequency, Computations Based on Period to Date, No Specific Date, and the Balances Method.  My understanding is the G/L Date in the header of the application dictates what G/L Date will be used in the journal entry when run in Final mode, and this can be advanced automatically (although it appears the option to not advance it if no balances exist does not work) to the next month's ending date.  What impact does the Thru Date on the Include Alloc processing option tab