Firewall Mode and Security Context Mode
The ASA runs in a combination of the following modes:
Transparent firewall or routed firewall mode
The firewall mode determines if the ASA runs as a Layer 2 or Layer 3 firewall.
Multiple context or single context mode
The security context mode determines if the ASA runs as a single device or as multiple security contexts, which act like virtual devices.
Some commands are only available in certain modes.