label, signature
簽 centers on marking something for identification or authentication: attaching a label or affixing a signature to confirm or designate.
簽 combines 竹 (bamboo) with 僉, which likely contributes the sound. Historically, bamboo slips were used for writing labels or documents, and the character came to mean a label or a signed document.
Bamboo 竹 was used to make slips for writing. The lower part 僉 looks like a gathering of people under a roof, suggesting a document that many people sign or label. Picture a bamboo tag with everyone's signature on it.
For セン, imagine a bamboo label signed with a hundred (百, ひゃく) names, but here use the on-reading of hundred: セン. The hundred signatures on the bamboo slip cue セン.
slip of paper bearing a book's title (pasted inside the book)