What does this stamp on my envelope mean?

When serving documents in an administrative proceeding, does the notice need to explicitly state what kind of document is being delivered—like "SUMMONS" or "DECISION"—to ensure it's received in hand?
Or is simply labeling it "ADMINISTRATIVE PROCEEDING" enough? Would failing to be specific constitute a procedural error?

I’d appreciate any insights or citations of relevant legal codes.

As far as I know, you don't have to put that on the envelope, and honestly, you shouldn't either. All you need is the sender and receiver info.