Hi Joris,
Yes, you are correct. The Bulk Email field on the Contact is set to Do Not Allow when a Contact globally unsubscribes and this field is then reverted to Allow when that Contact opts in again.
If you go to Marketing > Unsubscribes, you can view all of your leads’/contacts’ Unsubscribe records. These are active records that are updated when changes are made to their respective subscription preferences.
For example, if I have subscription lists for Monthly Newsletters and Weekly Sales Specials, and my Contact John Smith globally unsubscribes from all of my communications, an unsubscribe record will be created. If John Smith decides he would still like to receive some correspondence from me and opts back into my Monthly Newsletters subscription list, then the Unsubscribe record will continue to exist. Now, when you navigate to Subscription Lists in the related records menu, you will see that John Smith is still unsubscribed from the Weekly Sales Specials list. If he opts back into both lists, the Unsubscribe record will be removed all together.