What was destroyed. A description of the materials, documents, drives, devices or products, so there is no question about what the job covered.
How it was destroyed. The method used, from shredding paper to shredding drives and destroying product beyond reconstruction.
When. The date and time of destruction.
Chain of custody. Who held the materials from collection to destruction, with every handoff recorded.
Serial numbers, on request. Each hard drive or device itemized by serial number, so your asset register can be closed line by line.
Video, on request. Footage of the destruction itself, for when a written record is not enough.