Anyone know what table stores user subscriptions to various order types, and if necessary which application one would use to manually remove a users notification subscription for specific orders?

Currently digging into an issue a user brought up, he indicates he's getting email notifications for orders that he did not subscribe to, or create himself.
I would like to take a look myself, but for the life of me cannot find where requisition, or purchase order subscriptions are stored on a per user basis. I have scoured the forum here hoping someone may have inquired into this at some point and I've come up empty handed.

Any help or insight is appreciated.
Thanks,

HTK