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Customers can be set up so that payments auto apply to invoices, or the lab can manually apply payments against specific invoices.
Auto
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Apply
1) Auto apply would be used when a customer uses a "balance-foward" method, where they pay an amount (based on their statement), and the amount is applied against invoices, starting with the oldest invoice that has any balance due, until the payment is exhausted. This is what most customers will use for most labs.
2) Auto apply would not be would not be used when a customer pays on a "per invoice" basis (also sometimes called open-item). In this case, the customer will indicate (on the statement or cheque stub) which specific invoices they are paying, and will require the lab to allocate the payment against those specific invoices.
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