Gears of War: Judgment

The declassified missions in Gears of War Judgement are basically the optional challenges you can complete in almost every chapter of every act of the game. These challenges vary from section to section and scale of difficulty.

Gearspedia from Fandom by Wikipedia provides a brief explanation of what these mission are and where/what they are. For this you have to only do twenty of them but, there are more. The achievement "Quality Soldiering" pertains to completing them all then, completing them all on Inane difficulty will reward you with "Seriously Judgmental."

https://gearsofwar.fandom.com/wiki/Declassified_Missions