A tenant database backup consists of a full backup and any subsequent incremental backups.
If a backup includes incremental backups, then the backup's expiry date is always equal to the expiry date of the most recent incremental backup.
A full backup of the database backs up the database files and the transaction log.
An incremental backup completes faster than a full backup because it only backs up the transaction log. You must have a full backup before you can create an incremental backup. Incremental backups can be interspersed between full backups.