Blue money icon?

[Proud Black Women]

does anyone know what the blue circle money icon above my tac shooter is?

[PichuYang]

Primary Monkey Knowledge. Come on everybody!
When there is only Tier 2 below Primary on field, they attack 5% faster. If there is only Tier 3-4 Primary on field, their upgrades, including Tier 5 is 5% cheaper.

The bonus is lost when there is a Tier 5 Primary. The Discount is one of an interesting way to understand about money saving.