From Halopedia, the Halo wiki
Tactical Condition, sometimes referred to as the TAC CON or the Combat Alert, is the state or readiness aboard a UNSC ship or combat station. The higher the Tactical Condition, the more prepared the crew must be to fight enemies. At its highest level, every single crew member is presented with a firearm and reports to a designated action station.
In September 2552, Captain Jacob Keyes ordered his ship, the UNSC Pillar of Autumn, to Combat Alert Alpha when it arrived at Installation 04 and discovered Covenant forces there. It can be assumed that he was referring to TAC CON Alpha 1.
One month later, Cairo Station automatically went to TAC CON Alpha 2 when Covenant ships were detected in Earth's orbit, but it was quickly elevated to Alpha 1 when it was discovered that the Covenant were sending R'sisho-pattern Tick boarding craft to plant a bomb within the station.
Known Tactical Conditions
TAC CON Alpha 2: The second highest tactical condition. Tac Con Alpha 2 is ordered when the ships are preparing to engage in vessel to vessel combat with enemy combatants, usually, on a major scale.
TAC CON Alpha 1: The highest known tactical condition. Tac Con Alpha 1 is only ordered when the ship's crew must be prepared to engage in direct combat with enemy invaders. This is rarely the case, and usually only when the enemy is using boarding craft to enter the ship and attack its crew.
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