In the meantime, can someone explain what is the difference?

When an object (like a task) is updated (for example, someone comments on it), all the members of projects related to the object in any way potentially receive a notification.

For example, suppose task T123 has "Subscribers" @alincoln and #ops. Its "Projects" field is #infrastructure and #hardware.

When someone comments on the task, these users will be notified:

• @alincoln will be notified, because he is a subscriber.
• Subscribers to #ops will be notified, because it is a subscriber.
• Watchers of #infrastructure or #hardware will be notified.

Note that subscribers to #infrastructure and #hardware will not be notified. They are only notified when those projects have a direct/primary relationship with an object.

I see! Thank you. Very interesting.

I only realized last week that you can add projects to CC, and was wondering what that meant. Now it's clear.

Meanwhile, the "Subscribe" option doesn't exist anymore. What happened? Do users become subscribers just by joining a project? (that would make sense)

It's only available if you're a member. (This is inconsistent with other applications, but other object types don't have subscription restrictions, so I think it was easier to just remove the option.)

T6183 has the reasoning on this.

It should probably always be visible, but show you a message like "You need to be a member of this project to subscribe to it. Do you want to join?" if it's possible to join it, or "You need to be a member of this project to subscribe to it, but don't have permission to join." if it isn't.

This is probably not difficult technically, but I don't have a particular product solution in mind so I can't spell out how a contributor would approach it.