In order to understand how traffic flows through the segregated contexts it is important to understand how the ASA determines the context in which it will send the packets. This process is known as classification. The ASA uses the following criteria for classifying packets:
Unique interfaces: with this if only one context is associated with the ingress interface, then the ASA classifies the packet into the context.
Unique MAC addresses: Unique MACs are used when multiple contexts share an interface.
NAT Configuration: if unique MACs are not enabled, then the ASA will use mapped addresses in NAT config to classify packets.
Important note: If a destination MAC address is a multicast or broadcast MAC, then the packet is duplicated and delivered to each context.
Classification example: The same outside interface is shared so the classifier relies on unique mac address to properly pass the packet to the right context.
See the asa tag for additional ASA related posts, Cheers!