SMS Opt-in Keyword vs Opt-out Keyword: What's the Difference?

An opt-in keyword is a word a recipient texts to a business's number to give explicit permission to receive further messages - the specific action that establishes consent to be messaged in the first place.

An opt-out keyword is a word a recipient texts to stop receiving messages - most commonly STOP, though alternatives like UNSUBSCRIBE, CANCEL, or END are typically supported too, with HELP as a related keyword that returns support information rather than unsubscribing.

The two aren't symmetric in how much flexibility a business has over them. An opt-in keyword can be chosen and customized by the business - a campaign might ask people to text a specific word to join a list, and that word is up to the business to define. An opt-out keyword has no such flexibility: STOP specifically must always work, regardless of how a business would otherwise prefer to handle unsubscribes, and every messaging gateway is required to honor it. Once someone sends an opt-out keyword, a business must send a confirmation of the unsubscribe and cannot message that number again unless the person opts back in through a new, separate opt-in action. This asymmetry exists because opt-out compliance is a legal requirement, not a courtesy - a business can design its opt-in flow however it wants, but it has no similar discretion over honoring an opt-out once one is received.