The inbox is per directory. It holds notifications about work, and the newsletters the directory subscribes to.
Notifications
Tasks finishing, tasks needing input, errors, event reminders, issues assigned to you, issues resolved.
Newsletters
If the directory subscribes to newsletters, issues arrive here. They are fetched periodically and read in a panel beside the list.
Newsletter content is rendered in a sandbox, so a marketing email cannot run anything against your workspace.
Filters
All, notifications, newsletters, filtered.
Marking as read
Opening the inbox marks what you have seen.
Controlling what arrives
Per-category switches across three channels — push, feed and toast — under Notifications.