AutoCash Rule Set.

Receivables provides five AutoCash rules that we can use to create our own AutoCash rule sets. When we run Post QuickCash to apply customer's receipts, Receivables tries to use each AutoCash rule within an AutoCash rule set. If the first rule in the set does not find a match, Receivables uses the next rule in the sequence, and so on until it can apply the receipt. Auto cash rule set is used to apply receipt amount against invoices. The following are the five Rules: 
             1. Clear the account 
             2. Clear the past due invoices 
             3. Clear the past due invoices grouped by payment terms 
             4. Match payment with invoice 
             5. Apply to the oldest invoice first.
The importance of the five rules are explained below.
Clear the Account: Post QuickCash uses this rule only if your customer's account balance exactly matches the amount of the receipt. If the receipt amount does not exactly match this customer's account balance, Post QuickCash uses the next rule in the set.
Apply to the Oldest Invoice First: This rule matches receipts to debit and credit items starting with the oldest item first. This rule uses the transaction due date when determining which transaction to apply to first.
Clear Past Due Invoices: This rule is similar to the 'Clear the Account' rule because it applies the receipt to your customer's debit and credit items only if the total of these items exactly matches the amount of this receipt. However, this rule only applies the receipt to items that are currently past due.
Clear Past Due Invoices Grouped by Payment Term: This rule is similar to the 'Clear Past Due Invoices' rule, but it first groups past due invoices by their payment term, and then uses the oldest transaction due date within the group as the group due date. When using this rule, Receivables can only apply the receipt if the receipt amount exactly matches the sum of your customer's credit memos and past due invoices.
Match Payment with Invoice: This rule applies the receipt to a single invoice, debit memo, or chargeback that has a remaining amount due exactly equal to the receipt amount.
If none of the AutoCash Rules apply, the remaining amount is placed as Unapplied or On-Account depending on the Remaining Remittance Amount option specified in your AutoCash Rule Set.