A Frame Source consists of a source field as well as a source channel.
A Frame Destination consists of a destination field as well as a destination channel.
Sources and destinations are defined in gmsg.h.
Common sources and destinations
| Source / Destination | Description | Meaning of source/destination channel |
| SD_SERVER | Server program | Instance of the server program. Currently always 1. |
| SD_CARD | Hardware Channels | Channel ID |
| SD_CLIENT | Client connection | Server-assigned Client ID |
| SD_SCHED | Message Scheduler program | Scheduler instance. Currently always 1. |
| SD_SCRIPT | Script Processor program | Script Processor instance. Currently always 1. |
| SD_PGM | Program Loader | Loader instance. Currently always 1. |
| SD_BLM | Bus load monitor program | Monitor instance. Typical value is 1. |
| SD_USDT | USDT protocol layer | Channel ID. |
| SD_FLIGHT | Flight recorder application | Recorder instance. Typical value is 1. |
| SD_RESP | Message responder program | Responder instance. Typical value is 1. |
| SD_IOPWR | I/O and Power Interface | Interface instance. Typical value is 1. |