Internal means done within the firm, e.g. EQCR for a listed client. External means of the audit firm by someone outside of the firm. For example, in the UK, the Joint Monitoring Unit (JMU) provides independent feedback on the quality review processes of audit firms.
The Code says “Providing tax return preparation services does not usually create a threat” – that’s for all clients – so doesn’t need specific safeguards for PIEs.