General facts about signatures in Lotus Notes...

In fact, when we speak of signatures in Notes, it is effectively a cryptographic hash function: http://en.wikipedia.org/wiki/Cryptographic_hash_function.

From wikipedia: "A hash function takes a long string (or message) of any length as input and produces a fixed length string as output, sometimes termed a message digest or a digital fingerprint."

So, when we sign a Design Note in Lotus Notes, it takes the content of all the items of the Design Note marked as to be part of the signature, and builds a digest of them with the current date and the current user digital information, and places that digest in a specific item, "$Signature".

To know if an item is part of a signature, a flag is set to it, saying if the item has to be included or not. Some items must be part of a signature across the design family. If these items are not part of the signature digest, then an error saying that the Design Note has been corrupted will be triggered.

From this perspective, we can see that a Design Note is always signed globally.

Rare Exception

The only case where a signature (so a digital digest) must be inserted directly with the piece of code itself (like a section, a hotspot/button), and not at the Note level, is when we are talking about a Document (and not a Design Note) that contains its own code in a Rich text (for example, an email with a button).

In this case, since the document itself is not signed, the only way to "certify" the origin of the code is to put a signature digest with the code itself. This is needed in extremely rare occasions, since all code are usually found in the Form or the SubForm itself.

The only way to know if a Document needs this kind of signature is to parse all the Rich text items in the Document, something that can be extremely time consuming on a large database. Launching the process without checking the need for such signatures, will have a direct impact on the re-saving of all documents on a database (the last modification date, last editor name, among others, will be changed with values that do not reflect the effective use of the application).

Signing a Document on the Note level (with a $Signature item) is not something that usually needs to be done, unless it is for a security reason (like an authentication signature of an email, to prove that the email has not been altered after being saved initially by the author).