Every map in Disgaea D2 has prizes attached, in the form of the Bonus Meter. This is a randomly determined list of up to 10 rewards you can earn for performance on the map. The higher the map’s Bonus Rank, the better the prizes are. You can check this list at any time during your turn by selecting the Bonus option from the menu.
These bonuses can include EXP, which is handed out equally to every character who is still alive when the stage has been cleared; HL; consumable items like food or Fairy Dust; goods you can use as bribes in the Dark Assembly; or equipment, including rare- or legendary-quality weapons, armor, and consumables. You’ll receive any prizes you’ve earned upon clearing the stage, at which point they’ll show up in your Items menu.
Any successful attack partially fills the Bonus Meter, but advanced tactics are rewarded with increasingly large bonuses. Any method by which you can get multiple extra actions in a turn is a good way to fill the Bonus Meter, which includes Combo Attacks, Counterattacks, Cover Attacks, and long Geo Chains. The more hits you can rack up, the better the Bonuses get.
The Bonus Meter maxes out at level 9, at which point it cannot be filled any further and you’ll receive all the listed rewards upon clearing the map.
You can pass a Bonus Items, Please bill in the Dark Assembly that automatically fills the Bonus Meter to level 3 at the start of the next stage you enter. This is something of a gamble, as the Bonus Meter and its contents are always randomly determined and as such you won’t know whether or not the rewards will be useful. You pays your Mana, then, and you takes your chances.
The quality of the rewards in a given stage is determined by its Bonus Rank, which is listed on the stage selection screen. While there are several different ways to make the enemies on a stage more powerful, the Bonus Rank is fixed and cannot be changed.