What it is for
Scanning opens the phone’s messaging app on a new message to your number, with your text already in the box. It is the lowest-friction way to collect a reply from someone who is standing in front of a sign: no app to install, no account, no data connection needed beyond mobile signal.
The classic use is keyword opt-in — a poster that says "text JOIN to hear about gigs" becomes a code where JOIN is already typed. It is also useful for reporting: a code on a bin, a bus stop or a broken machine, with the asset number prefilled so you learn which one without asking.
The format, and why it varies
This site writes SMSTO:number:message, which is the format with the broadest scanner support. You may also see sms: links, and some readers prefer that spelling, but SMSTO is what most camera apps handle correctly when a message body is involved.
Include the country code for anything that might be scanned by a visitor. The tool removes spaces, dashes and brackets for you but keeps a leading plus, so pasting a number in any readable format works. Short codes work too — just type the short code as the number.
What to warn people about
The message is not free for the sender. It goes out at their normal rate, and premium or short-code numbers can cost noticeably more, so say what the message costs anywhere the number is not obviously a normal mobile.
Nothing sends on its own: the person presses send, and can edit the text first. That means the keyword you prefilled can be changed, so do not rely on it as a password or as a way of identifying who scanned.
What goes inside the code
The code contains one short line:
SMSTO:+34600123456:JOIN- the number keeps a leading
+; spaces and dashes are stripped - everything after the second colon is the prefilled message
- leave the message blank and the code opens an empty conversation
Before you print it
- Keep the keyword short and unambiguous — it is what people will retype when scanning fails.
- Include the country code if strangers or tourists will scan it.
- State the cost near the code when the number is not a standard mobile.