What actually goes into a strong affidavit or declaration, beyond a reference letter

Had a situation where a piece of evidence needed a sworn declaration rather than a standard reference letter (attesting to a fact that wasn't otherwise documented anywhere), and want to share what I learned about the difference, since I'd conflated the two before this came up. A declaration is a formal, sworn statement of fact — typically used when you need to attest to something specific that isn't captured in an existing document, rather than a general endorsement of your work. It needs precision: specific, factual, checkable statements, not general praise or opinion, and typically needs to be properly executed (signed, dated, with the appropriate formal language) to carry the weight it's meant to. Where I used one: to establish a specific factual detail about a role I'd held that wasn't otherwise documented in any surviving company record (the company had since closed). The declaration came from a former colleague who could personally attest to the specific facts. This isn't a replacement for a reference letter praising your work — it's a different tool, for establishing specific facts that need attesting to rather than general recognition that needs describing.