When you save a receipt the amount entered in the Allocated column against each line is used to adjust the amount outstanding on the corresponding invoice, credit note or on account payment.
For example:
Allocating £45 to an invoice with £45 outstanding will mark the invoice as paid
Allocating £50 to an invoice with £75 outstanding will leave £25 outstanding balance - the invoice will not be marked as paid
Allocating -£70 to a credit or an on account payment of -£70 will leave nothing outstanding on that item