I'm not a mattermost developer, but the way I understand it is like this:
- Bold: People are talking in that channel since last I opened it
- Number: Someone is talking directly to me or mentioning me in the conversation
That's why privates have the unread notification because the message is always direct to you
In our organization we have several channels depending on people roles, so instead of sending a message in Town Square to multiple users, I send it the "Database" channel (if I want to notify those involved in the database)
But I guess each organization has different needs.