Messages in emforth are integer constants. Unlike regular constants - messages are pushed onto the message stack not the data stack. The words >M and M> can be used to move to and from the data stack.
The messages are spilt into two types - "messages" and "gmessages". The difference is "messages" are automatically assigned to a number when created - gmessages require the user to specify the numeric value when the are created.
Messages (and gmessages) are used for communication on a single processor - the actual numeric value is unimportant as long as they are unique.
Gmessage are intended for communication between processor - every processor needs to be using the same values for each message.