A message set is a container for grouping messages and associated message resources (elements, types, groups).
A message set is a folder in a message set project that contains a messageSet.mset file. The name of the folder is the name of the message set. A message set project can contain just one message set.
When you create a new message set, a new message set project is automatically created with a name that is, by default, the same as that of the message set.
You can base your new message set on an existing message set. If you do so, all of the definitions in the existing message set are copied into the new message set.