The ALTER TABLE command is used to change the structure of an existing table, such as adding or modifying columns or constraints.

The START SLAVE; command is used to start replication on a replica server, allowing it to begin receiving and applying updates from the primary server.

MySQL Enterprise Authentication supports password policies, including enforcing password complexity, expiration, and history, which improves security by ensuring strong authentication credentials.

VARCHAR is the best choice for storing phone numbers because they often contain non-numeric characters like "+" or dashes, and are not used for calculations. INT or DECIMAL would not be suitable since they would strip these characters.

The SUPER privilege allows users to perform administrative actions, including starting or stopping the MySQL server, and other system-level operations.