Definition of 'Prepayment on Invoice' flag in apply payment view

Hi,

I have 1 standard invoice $2000 and 1 prepayment invoice $2000.

if i only click the 'Apply' flag in apply prepayment view, the standard invoice total amount will reduce to zero.

but if i click 'Apply' and 'Prepayment on Invoice' flag in prepayment view, the invoice total amount will be 2000 and distribution total will become to Zero.

May i know what the meaning of 'Prepayment on Invoice' and what is this function use for?

Thanks,

Roy