To manually assign a MAC address to this interface, on the Advanced tab enter a MAC address in the Active Mac Address field in H.H.H format, where H is a 16-bit hexadecimal digit. For example, the MAC address 00-0C-F1-42-4C-DE would be entered as 000C.F142.4CDE.

If you share an interface between contexts, you can assign a unique MAC address to the interface in each context. This feature lets the security appliance easily classify packets into the appropriate context. Using a shared interface without unique MAC addresses is possible, but has some limitations.

http://www.cisco.com/en/US/docs/security/asa/asa80/asdm60/user/guide/contexts.html#wpxref64049

http://www.cisco.com/en/US/docs/security/asa/asa80/asdm60/user/guide/ifcmulti.html#wp1055358