“ISA 530 Audit Sampling requires auditors to determine a sample size that would reduce the sampling risk to an acceptably low level and to select items in the sample in such a way that each sampling unit in the population has a chance of selection.”
The individual items from which selection is made – e.g. invoices/GRNs for tests of controls or account balances in a list extracted from a receivables ledger. Or it could be each $ amount. (Monetary unit sampling is assumed knowledge of AA and you will find an illustration on page 65 of the AA notes.)