SMS signature
An SMS signature is the bracketed sender name at the start of a China-bound template and part of the approved content.
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An SMS signature is the sender name in brackets at the head of a China-bound message template. It names the responsible sender and is part of the template approval.
The signature is content, not metadata: it travels inside the message text and counts toward its length.
Rules in practice
The signature has to match the registered sender, use the accepted bracket style and sit at the front of the message. It counts toward the length, so a long signature leaves less room for the body. A different sender name needs a new registration and a new approval.
The operator may reject a template whose signature does not match the entity, whose content falls outside the approved category, or whose variables could change the meaning of the message. The China SMS template checker checks the signature, template and length before submission.