There are various ranks that you can unlock in The DioField Chronicle. One of them is the Meal Rank which grants various buffs to your units during missions and combats. However, you need to unlock it first by completing a subquest.
The subquest you need to complete to unlock Meal Rank is called A Meal Fit for Mercenaries. You can activate this quest by talking to the Dining Hall Worker NPC. You can find him in the kitchen which is in the upper left corner of Chapter 2.
After receiving the quest, you’ll then need to start the Secure the Supply Route mission. For this, you’ll need to defeat enemies along the way. Once you managed to complete this mission, just go back and speak to the Dining Hall Worker.
After the dialog, it will then complete the quest, unlocking the Meal Rank at the same time.
With the Meal Rank, you can reduce the damage that your units received during battle by a certain percentage. There are also bonuses that increase their overall damage. Lastly, you can also benefit from an increase in the amount of XP you gain on missions.