MAVLink messages are defined in XML files in the mavlink/message definitions folder. The messages that are common to all systems are defined in common.xml (only messages contained in this file are considered standard messages). MAVLink protocol-specific messages and vendor-specific messages are stored in separate XML files.
The common messages are provided as human-readable tables in: Common.
Vendor forks of MAVLink may contain messages that are not get merged, and hence will not appear in this documentation.
The human-readable forms of the vendor XML files are linked below: