There are two types of call agreements: (1) when the call is natural, the agreement is said to be a treatment, and (2) when the call is artificial, the agreement is said to be a convention.
The purposes of announcements and alerts are similar, but an announcement gives the meaning where an alert may prompt the opponents to ask the meaning.
The player who made the call may contribute to its explanation only after the auction and only if he/she is declarer or dummy.
Slightly different rules apply when screens are in use.
Sponsoring organizations set rules on which calls should be announced.
The ACBL specifies announcements including "Transfer" for some transfer replies to notrump bids, the point range such as "15 to 17" for an opening bid of one notrump, and "Forcing" or "Semi-forcing" for a 1NT response to a major suit opening bid.