Sender Authentication Package vs. Private Domain
What is the difference between a Private Domain that is included with the Sender Authentication Package (SAP), versus a Private Domain that is purchased separately?
A Sender Authentication Package is a "branding" tool. A Sender Authentication Private Domain includes link-wrapping (i.e. links, images, and anything else that would normally reflect Marketing Cloud will reflect the chosen Sender Authentication domain instead when you hover). The domain is also authenticated with SPF/Sender ID and DKIM/Domain Keys.
A private domain is used as an authentication tool. A Private Domain that is purchased separately for sending is an authenticated domain for use in the FROM address only. It is set up either via a User or Sender Profile, and this type of Private Domain DOES NOT include link or image-wrapping. It has an MX record, SPF/Sender ID and DKIM/Domain Keys.
While a single account MID can support only 1 SAP for branding purposes, there can be multiple domains in an account.
For more information check out our Getting Started with Marketing Cloud Email Studio resource hub.