The SPA logical interface is the conceptual boundary through which components are able to participate in a SPA system. Unlike a physical interface which defines connectors@ pin-outs@ and signaling levels@ the logical interface consists primarily of messages. This document describes the messages that are passed through a SPA system@ the circumstances under which those messages flow@ and the protocols for sequencing those messages. This document does not attempt to describe how the messages are transported from one component to the other. This document and its messages are agnostic to the message routing@ message delivery@ or topology of the physical layer of the network. These details are documented in the SPA Networking Standard (AIAA A-133-2-2013) and its associated documents. Furthermore@ this document does not attempt to describe the details on how these messages are implemented for a mission. The use of these messages can seem vague without a complete implementation or mission-specific example. The SPA Guidebook (AIAA G-133-1-2013) gives examples and principles on how these messages are used to achieve the capabilities of fully functional plug and play architecture.