Its very simple ...
You reverse a receipt when no payment was received from the customer for the receipt amount.
Reversing the receipt creates reversal journal entries in the general ledger and reopens all of the debit and credit items that were closed by the original receipt.
You unapply a paid receipt either to return payment to the customer or to reapply a receipt applied in error to the correct transaction.
If you unapply a receipt to return payment to the customer, either with a refund or an on-account credit, you must create a credit memo against the original transaction that was closed by the receipt application.
You can delete manual receipts that were created but not yet applied to transactions.
You can delete automatic receipts belonging to an automatic receipt batch that has not yet been approved. When you delete a receipt from a batch, the transactions closed by the receipt become available for automatic receipt selection.