who command: What do the - + ? symbols represent with the -T option?

Through the man who appears the following for the -T option -T, -w, --mesg add user's message status as +, - or ? According with some tutorials, if is executed who -T should appears these symbols. In my case - appears for all the users logged through tty and + for all the users logged through ssh - nothing about ? What do the - + ? symbols represent with the -T option?