Which type of data extension has a send relationship and adds contacts to All Contacts when you send to them?

The type of data extension that has a send relationship and adds contacts to All Contacts when you send to them is the Sendable Data Extension. This data extension is specifically designed for sending marketing communications, meaning it is linked to the contacts in your Marketing Cloud account.

When you use a Sendable Data Extension, the contacts within it are considered eligible for email sends and are included in the Contact Builder's All Contacts list after the communication is deployed. This relationship ensures that the contacts are now part of your overall audience, allowing for better management and segmentation of your subscriber base.

In contrast, other types of data extensions do not have this specific relationship. For example, a Nonsendable Data Extension does not permit sending messages directly to contacts within it, hence it cannot contribute contacts to All Contacts. Transmittal Data Extensions and Attribute Data Extensions serve different purposes (like storing data or attributes about contacts) but do not facilitate the process of adding contacts to All Contacts after a send. Thus, the Sendable Data Extension is the only type that meets the criteria outlined in the question.