>proofs
If we're building a case we might want to adopt a numbering system for evidence. We won't possess physical evidence, but we may and do possess digital evidence (e.g., the photo at SOTU; certain pdfs, etc.).
>map
If we're building a map we might want to adopt a numbering system for persons of interest. This avoids duplication, like a database primary key, and facilitates connections to other persons and/or evidence.
Just some techniques used by those who do criminal investigations for a living…