Plus Bonus

Sometimes, weapons or summons will come with a +1 on it. This bonus adds +1 HP and +5 ATK to whatever has it. The bonus can be transferred by either using it directly as upgrade fodder, or by resetting bonuses. The maximum bonus is +99. The bonus can even be applied to weapons or summons already at their current level cap.

Farming Bonuses
All draws have a chance of yielding a +1 bonus, which is a major reason it is recommended to perform all 100 Rupie draws each day. Occasionally, the current draw promotion will cause all Premium Draws to come with a +1. This can be a major source of bonuses if you are willing to spend money and/or forgo increased rates from Premium Gala to draw during these promotions.

High Level raids also have a chance to drop Mirage Munitions and Mirage Makers directly.

Resetting Bonuses
If a +1 comes on an item that you do not want to lose, you can reset its bonuses, which creates a fodder item with the +1 that can be used in upgrade. Mirage Munitions are weapon fodder, Mirage Makers are summon fodder.

From the Inventory screen, select the weapon or summon with the bonuses, then scroll down to the buttons at the bottom. Click Reset Bonus, which will display a confirmation screen with the cost in Rupies to reset the bonus. The cost is 500 Rupies for a +1 bonus, and 300 + (300 * current bonus value) Rupies if the current bonus is greater than 1. The mirage item is then sent to your crate.