The MIDI sequencer specific meta message carries information that is specific to a MIDI sequencer produced by a certain MIDI manufacturer.
This message belongs to the category of MIDI meta messages. Since this is a meta message the MIDI event that carries this message may exist in MIDI files, but it is never sent over MIDI ports to a MIDI device.
This message consists of a variable number of bytes of data. The first byte is the status byte 0xFF, which shows that this is a meta message. The second byte is the meta message type 0x7F, which shows that this is a sequencer specific meta message. The third byte is the length of the message (the number of remaining bytes). The fourth byte specifies the manufacturer ID and hence prompts a device made by that manufacturer to do something and usually prompts devices made by other manufacturers to ignore the message. Other bytes may follow.
A list of know manufacturer IDs is provided under the topic on MIDI system exclusive messages. For system exclusive messages, and presumably for sequencer specific meta messages, manufacturers that do not have an assigned ID may use the IDs assigned to other manufacturers assuming that they do not change the structure of the message.
The following is an example of a sequencer specific meta message.
0xFF 0x7F 0x04 0x41 0x04 0x01 0x56
The status byte is 0xFF, which means that this is a meta message. The meta message type is 0x7F, which means that this is a sequencer specific meta message. The third byte is 4, which means that there are four remaining bytes. The fourth byte 0x41 is the manufacturer ID of Roland, which means that this meta message is designed by and for Roland. The remaining bytes prompt a Roland sequencer to do something (note that this is not an actual Roland message, but a made-up example).