An exhibit is only as good as the account of where it has been since it was picked up. That account is called continuity, or chain of custody, and it is the thing most often attacked and most easily lost.
What a complete record says.
- What it is, described as found rather than as assumed. "A dark stain on the left sleeve", not "blood".
- Where it came from, precisely enough to be pointed at on a plan or a photograph, and by whom it was recovered.
- When, to the minute.
- Every hand it has passed through since, each transfer signed and timed by both the person handing over and the person receiving.
- Where it has been stored, and under what conditions where those matter — refrigerated, dry, sealed.
How it is lost.
- An unsigned transfer. A gap of two hours nobody can account for is the gap the defence will spend the trial on.
- A seal broken and not recorded. Every opening is an event and is written down, including the ones with a good reason.
- Two items in one bag. Cross-contamination cannot be undone and cannot be disproved.
- A label written later "when there was time". Later is when the detail is already wrong.
Contamination is a two-way street. The recovering officer's own DNA, fibres and prints reach the item as readily as anyone's. Gloves changed between items, not between scenes. Nothing on a scene is eaten, drunk, smoked, or set down and picked up again.
Source. National Institute of Justice, Crime Scene Investigation: A Guide for Law Enforcement (NCJ 178280), https://www.ojp.gov/pdffiles1/nij/178280.pdf ; UNODC, Crime scene and physical evidence awareness for non-forensic personnel, https://www.unodc.org/documents/scientific/Crime_scene_awareness__Ebook.pdf ; and the standards work of the NIST Organization of Scientific Area Committees for Forensic Science, https://www.nist.gov/osac
What this is not. Procedure. What makes an exhibit admissible is decided by the law of the jurisdiction and the standing orders of the service, and where they differ from this page, they govern.