Heroes contains new multi-faction hero cards. Invasion features new ships and bases with Double-Ally abilities. Requisition contains new ships and bases which cost less to acquire for each card of their faction you have in play. Tech features the new Tech card type, which grants you special abilities that you may activate on each of your turns.
each pack is one of the following 4:
High Alert: Heroes
High Alert: Invasion
High Alert: Requisition
High Alert: Tech