Any direct charge using a blanket item type will create a long-lived charge that can be invoiced multiple times.
Such a charge is considered open until its purchase order is "finalized" (received).
"Finalizing" a PO changes the PO’s state to received (assuming there are no pending lineitems on the PO) and fully dis-encumbers all blanket charges on the PO by setting the fund_debit amount to $0 on the original fund_debit for the charge.
Invoicing a blanket charge does the following under the covers:
Create an invoice_item to track the payment
Create a new fund_debit to implement the payment whose amount matches the invoiced amount.
Subtract the invoiced amount from the fund_debit linked to the original blanket po_item, thus reducing the amount encumbered on the charge as a whole by the invoiced amount.
A PO can have multiple blanket charges. E.g. you could have a blanket order for "Popular Fiction 2015" and a second charge for "Pop Fiction 2015 Taxes" to track / pay taxes over time on a blanket charge.
A PO can have a mix of lineitems, non-blanket charges, and blanket charges.
A blanket Invoice Item Type cannot also be a prorate type, since it’s nonsensical. Blanket items are encumbered, whereas prorated items are only paid at invoice time and never encumbered.
