I think Polina is just pointing out that the assignment extension is basically no more than a standard relation (between an entity type and the user type) but with the specific functionality of notifications laid on top.
So when a more generic notification system is developed, similar functionality will be possible for any number of entity<->user relations.
So I imagine it could be set up so with auto-notification rules like: “Notify X when (boolean)” and here X can be hard-coded, or can be any expression that evaluates to a User.
Notify Task.Watcher when (IsChanged(Task))
Notify Task.Owner when (Now = Task.DueDate)
Owner and Watcher are just normal entity<->user relationships
The nice thing about this is that standard entity<->user relationships can be defined as one-to-many, many-to-many or many-to-one.
Useful if, for example, I want to limit a task to having only one owner, or I want to limit a user to only be allocated to one department, or…