I a trying to understand the relationship between All Subscribers, Business Units and Business Unit Filters and how exactly the BU filter is applied. From my understanding, the BU Filter controls if a given BU can access a subscriber in the All Subscribers table in that BU. My BU unsubscribe setting are set to unsub from each BU, not the Enterprise account. I do realize that there is only one true All Subscribers Table for a given account and BU's actually have a hidden status field.
Let's say I'm using a Data Extensions to import new subscribers into child BU-1, and there is a BU filter on BU-1 that is a true or false value i.e. BU-1 == 'true'. If I import these new subscribers through a data extension and try to send them an email, will the send process evaluate if they meet that subscriber filter on send, or will the email still be sent to those subscribers, then they'll join the All Subscribers table.
My follow up question would be in that scenario, would I then need to run an automation to update the field value of each subscriber in the All Subscriber's table, such that they will satisfy the BU Filter condition i.e. BU-1 == 'true'.
I am in the process of setting up multiple BU's and wondering if the BU filter is actually used as a 'send permission' or status value for each BU, or if it's more of a UI tool so that marketers can't see data from other subscribers not belonging to the BU they work in.
Thanks and any clarification is helpful!