An official battle is neither a training battle against an ally or a replay battle video, but a battle against a random mechatar . Each of your Mechatars has three bonus matches, which give extra experience to your mechatar.
The first bonus match is your mechatar against a mechatar of one level lower.
The second bonus match is a mechatar of the same level as yours.
The third match is a mechatar a level higher than yours.
Upon losing such a match, you still gain XP for your Mechatar, but you can gain more by winning.
If your Mechatar is level 10, your mechatar will first face a level 9, then a level 10, and then another level 10.
If your Mechatar is a recruit, it will face a level 4,5,6 or seven and you will be guided through.