Client-side signature
A signature stored in the email app itself, such as Gmail settings or Outlook, and added while you write.
A client-side signature lives in the sender’s email app: Gmail settings, Outlook, Apple Mail or a phone app. A management tool can write it there centrally, for example through the Gmail API sendAs settings, without routing any mail.
The sender sees the signature while writing, and replies keep it under the new message. The limit is coverage: mail sent from an app that has no signature set, such as some mobile apps, goes out without it. Sendoff works client-side.