In an effort to document the set up of the All Staff space, I would like to know exactly which permission or setting enables all internals users to receive email notification every time something is published in the All Staff space?
This can depend on a user's specific user preferences which they have defined for themselves in their user menu >> preferences tab. For a user to receive an email notification 2 this will need to be in place.
1. The user will need to be following the space or content (so that an update is added to their Jive inbox)
2. The user will need to have their user preferences configured so that this action (an item being added to their Jive inbox) also triggers an email notification.
Additionally, some of this can be automated (depending on your Jive version) through the use of an auto-follow add-on Jive Auto-Follow Extension or the News feature (available in Jive 8 and Cloud) - Core Help - Customizing News
Also, are people able to unfollow this space or opt out of email notifications? HR wants to ensure that everyone is being notified of all content published within our All Staff space.
If this is being managed by a news stream or auto-follow add-on the user would not be able to "un-follow" the content and could receive email notifications, depending on how the feature was implemented in your instance