I have a Master BU and 5 child BU's. When a subscriber signs up on the website they were added to child BU and a welcome Email is triggered, they were added to All subscribers in all child BU'S and Master BU's. If I unsubscribe from the mail child BU, they are still showing active in other Child BU's. Is there a way that we can set up that will be adding to that Child BU only not to all other BU's I did check the trigger under subscriber management it is adding to the correct list and there are 2 check boxes "Add Subscriber to list" and "update Subscriber" I did also check the Define Subscriber filter, there is one Filter as Reduced Frequency Is YES Do I need to change anything here?
There is only one All Subscriber list that is shared across different BU's on your instance.
To keep the Unsubscription status processing at global level, meaning it will reflect the status the same between all the BU's, change the Unsubscribe Settings to be
Subscribers will be unsubscribed from all business units in the Enterprise for every BU.
Keep in mind this change will not work retroactively for the subscribers that have already unsubscribed.
The other point, if you need to adjust and keep the visibility of subscribers between BU's private, you will need to enable Subscriber Filter based on a subscriber attribute. This means every individual subscriber will be visible on top-level BU and the sub-BU level where it matches the filter criteria.
The most common solution is to create a new subscriber attribute like
MID and use values specific to that BU as filtering rule.