jForum has two "watch" options, both of which are designed to allow users to opt-in but not really designed to have all user "opt-out".
First, users can decide to "Watch" a forum for new topics by clicking on the "Watch this forum" option in the main forum list. If they turn this one, they will receive an e-mail anytime a new Topic is created (but not Replies).
There is currently no code to automatically set this watch flag for all users. The individual user must select this.
The other option is to watch a particular Topic for replies. The option you talk will set the "watch this topic" option in the Post form to be enabled. This means that any poster who doesn't uncheck this will get a e-mail if a reply is posted.
However, if someone hasn't posted to a topic thread, they will have to manually "opt-in" by clicking "Watch this topic".
As with Forums, there is no code to force every topic to be watched by all users.
I hope this answers your questions or points you in the right direction. [originally posted on jforum.net by monroe]
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