-- **type** - a string that must be "subscribe"
-- **backlog** - an integer specifying how many previous locations to
- send for the start. Limit is per-imei.
-- **imei** - a list of 16-character strings with IMEIs of the
- tracker terminals to watch.
+- **type** - a string "subscribe", or a command for the terminal.
+- **backlog** - for "subscribe, an integer specifying how many
+ previous locations to send for the start. Limit is per-imei.
+- **imei** - for "subscribe", a list of 10- or 16-character strings
+ with IMEIs of the tracker terminals to watch, for other commands -
+ IMEI of the particular tracker.
+- **txt** - for "msg" command, text of the message to send to the
+ terminal, in UTF-8.