Alpha Class:

Computer operations that include Secret or lower systems, such as access to Power Control Systems, Computer Control Systems, and Weapon Control Systems - Ship's Command Staff Only

Beta Class:

Computer operations that include Restricted or lower systems, such as access to Maintenance Systems, Power Distribution Systems or Communication Systems - Regular users having passed security screening, aka. Starfleet Officers.

Gamma Class:

Computer operations that include sensitive or lower systems, such as Public Transportation Systems, Public Information Systems or Public Nutrition and Environmental Control Systems - Visitors and other transient personnel