Engagement - All Subscribers email address update behavior when sending to data extensions

Sendable data extensions contain a Subscriber Key field used to perform a lookup on the All Subscribers list, in addition to an email address field. 

When sending to a data extension, if:

Subscriber does not exist in All Subscribers:

The subscriber key is used to create a record in All Subscribers, and the email address from the data extension is used to populate the All Subscriber record's Email Address column. The send is delivered to this email address.

Subscriber exists in All Subscribers:

The send is delivered to the Email Address value in the All Subscribers record. The Email Address field in the target data extension will not be used to update the subscriber's record in All Subscribers. 

Journey Builder through it's default functionality will update All Subscribers at send time

Subscriber exists in All Subscribers (Salesforce Data Extension):

Sends to Salesforce Data Extensions prioritize the data extension's Email Address field over the All Subscribers value. If the value differs, sends are delivered to the data extension email address, and the All Subscribers record is updated with that address.