Ok the manual says "
Message users
The final communication method is similar to the email method, but instead of sending email, it will insert a new conversation with each user targeted by the criteria, containing the message you enter.
To prevent your own inbox receiving a deluge of replies, you may lock the conversations as soon as they are sent, which will essentially turn them into read-only messages.
If you do not lock the conversation, you can optionally leave the conversation and ignore future replies, or leave the conversation and accept future replies, which will discourage users from replying to the conversation even though they are able to do so."
So not much else to say, thank you for your help.