Yellow Alert (also Condition Yellow or Code Yellow) is the second highest alert signal status on Starfleet vessels, one stage below red alert. It designates a ship-wide state of increased preparedness for possible crisis situations. In this status, shields and deflectors are brought to full power; however the weapons systems remain off line.
A yellow alert may either be activated manually by any member of the crew in preparation for a tactical or environmentally hazardous situation, or automatically upon detection of unknown or hostile spacecraft, or upon detection of certain system malfunctions. If the situation escalates, a crew member or the main computer will order a red alert.
During yellow alert the on duty crew will prepare for possible combat or hazardous conditions. Any senior staff that are not on duty will be contacted and ordered to attend their positions.
Red Alert (also Condition Red or Code Red) is the highest alert signal status on Starfleet vessels, one stage above a yellow alert. In this status, weapons systems and shields are brought to full power, as the ship prepares for combat. Red alert is also ordered during various other emergency situations, such as a ship being boarded, radiation exposure, a security alert, massive systems failure, an imminent warp core breach, and an evacuation order among others.
The red alert may either be activated manually by any member of the crew or automatically when a ship receives damage or upon detection of certain system malfunctions.
During a red alert, lights are dimmed (to allow better viewing of the LCARs system), alarms blare, and computer terminals show constant red graphics. Upon activation of the red alert all crew members are required to attend their designated combat stations. All civilians will move to designated civilian ‘safe zones’.
Auto Destruct (also Self Destruct), is the last defense, this is usually done when there is a threat that stands a chance of capturing the vessel intact.
It takes the CO and at least two other senior officers to activate the Self Destruct sequence.