An SMS keyword is a word or short string that a person texts to a short code to tell the program behind it what they want. Advertisements usually print it in capitals next to the number it should go to, and the software on the other end matches the incoming message against the keywords it knows and answers accordingly. Most programs ignore capitalization, so join, Join and JOIN all land in the same place.
Keywords that sign a phone up
The best-known use is opting in. Texting a program’s keyword to its code is one of the recognized ways to agree to receive its messages, alongside typing a number into a web form or agreeing at a store counter. Under the industry rules in CTIA’s Short Code Monitoring Handbook, a recurring program then has to send one confirmation at once, naming the program, saying how to get help and how to quit, describing how many messages to expect, and warning that message and data rates may apply. One opt-in covers one program. The handbook forbids using a single opt-in to sign someone up for several programs, even when they all run on the same code, and it bans selling lists of opted-in numbers.
Keywords that fetch an answer
Plenty of keywords work as commands and subscribe nobody. Walgreens draws the line clearly on its prescription code, 21525: RXSTATUS returns one status update, while JOINRX signs the phone up for ongoing alerts. FEMA’s code 43362 answers PREPARE with preparedness tips, SHELTER plus a ZIP code with open shelters nearby, and LIST with every keyword it accepts, as Ready.gov sets out. The Postal Service goes a step further on 28777: a tracking number on its own brings the latest scan, a tracking number followed by a keyword asks a narrower question, and QUIET stops updates overnight until AWAKE turns them back on, according to its Text Tracking FAQs.
Reserved keywords
A few words belong to the subscriber rather than the program. Every short code program has to treat STOP, END, CANCEL, UNSUBSCRIBE and QUIT as an opt-out, reply with a single message confirming it, and then stop sending. Extra words, punctuation or capital letters around the keyword are not allowed to break it, so “Stop texting me” has to work as well as “STOP”. The handbook also tells providers to scan incoming messages for people who are plainly trying to leave with other wording and to remove them too.
HELP is reserved in the other direction. Whether or not the person texting is subscribed, a HELP message must bring back at least the program’s name and a way to reach customer care, such as a toll-free number or an email address. That makes HELP the safest first reply to an unknown code: it identifies the sender without agreeing to anything.
Keywords on shared codes
On a shared short code, where a messaging provider runs many clients’ programs on one number, the keyword does the sorting. Each client reserves its own words, and an incoming message is routed by whichever one it contains. Two clients cannot hold the same keyword on the same code, which is why keywords on busy shared codes often end up long or oddly specific, a brand name with a number attached. STOP works differently: the audit standards expect it to end every program the phone belongs to on that code, whichever keyword started them.
Since then, T-Mobile’s messaging code of conduct, revised in late 2020, has banned shared codes outright, and the brands that relied on them have had to move to numbers of their own.