Most guards are classified roughly according to their relative danger to the player. The game difficulty setting and mission difficulty level determine the number of guards of each class present at the start of a mission; each corporation's guards within each class are equally likely to appear, except that human guards appear very slightly more frequently than drones in Sankaku facilities.

Some enemy units are not classified because they appear depending on the mission type, regardless of the mission difficulty level:

• Captains appear in Detention Centers, some Security Dispatches and some Vaults.
• Financial Executives appear in Chief Financial Suites.
• Refit Drones appear in Drone Rescue side-missions.
• Scientists appear in Program Compile side-missions, and in the OMNI Foundry Lab mission.

Additionally, Enforcers and Elite Enforcers are not classified, because they are never present at the start of the mission.