Plan 99 was a pre-determined code phrase created by Clone Force 99. The plan consisted of a member sacrificing their life for the sake of their squad in the event of the squad's safety being in jeopardy.
Around 18 BBY on Eriadu, during the bombing of Raven's Peak, Tech executed the plan, sacrificing himself by shooting down the connection hinge that connected the two halves of the rail car, causing half of it to break and fall, to which he was attached, in order to let the rest of squad escape before the Imperial forces took down the entire rail car.
During The Battle on Mount Tantiss, Crosshair implied that he would execute Plan 99 by infiltrating the facility alone to find Omega so that she may be guaranteed to reunite with Hunter and Wrecker once she's safe. However, Hunter and Wrecker disagreed of the plan and the group infiltrated the base together. A plan that succeeded and ultimately granted them their freedom from being hunted by the Empire.