Level 1: Basic Operations:

The privilege to use a turbolift, a replicator, a heating and ventilation system.

Level 2: Public Databases:

The privilege to use a Public News Network and similar databases.

Level 3: Subspace Communications:

The privilege to access the standard subspace module for communication outside the ship.

Level 4: Restricted Databases and Lower system functions:

The privilege to access restricted databases, such as room distribution and cargo availability charts. Also lower system functions such as System Diagnostics up to level 3.
Enlisted Officers have this access, modified for their departments and sub-space messages are routed through Ops.

Level 5: Secret Databases and Higher System Functions:

The privilege to access secret databases, such as personnel records and defense status charts. Also higher system functions such as System Diagnostics up to level 5, and authorization commands for use of systems such as weapons and power control.
All officers and Starfleet ambassadors have level 5 clearance, modified for their departments.

Level 6: Full system functions:

The privilege to access full computer controlled systems and full diagnostics and shutdown authority.
All Department Chiefs have level 6 clearance, modified for their departments, while the ship's CO and XO have shipwide level 6 clearance as well as special command clearances.

Level 7: Special system functions:

On special authority, this level gives the privilege to monitor other communication within the ships connected systems, including the Secure Subspace Communication Modules. To gain this level of clearance, two different Level 6 users must together initiate the temporary clearance.