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Six characters, chosen once, and awkward to change later. Worth ten minutes now.
What the recipient sees is your six characters plus a hyphen and a category letter, added by the operator at delivery: METAND-P for promotional, -T for transactional, -S for service, -G for government.
You do not register the suffix and you cannot choose it. It is worth knowing because the header your customer sees is eight characters, not six, and because the same six characters appear differently depending on which category the message went out under.
Six characters is less than it sounds. The ones that work are usually a recognisable truncation of the business name rather than a clever abbreviation — the recipient has a fraction of a second to decide whether the message is legitimate, and a header they cannot connect to you is one they will treat as spam regardless of what it says.
Register more than one if you send genuinely different kinds of traffic. Registration is slow and free; discovering later that your transactional alerts go out under a header your customers associate with marketing is neither.
A header can be suspended upstream without anyone telling you, and every message sent against a suspended header is refused by the operator and charged anyway. We re-confirm every registered header with the operator daily and refuse to send against one we have not been able to confirm — you see its state in your DLT manager rather than finding out from a delivery rate.