An inspection and test plan sets out the quality verification for a scope of work: what will be inspected or tested, at what stage, against what criteria, by whom, and with what records.
Key stages are marked as hold points (work must not proceed until verified) or witness points (the other party is invited to observe). The completed ITP with its records becomes part of the quality evidence for handover.
Well-kept ITP records prevent the worst quality outcome: covered-up work whose compliance can no longer be verified without opening up.