I have a Mac app distributed in the Mac App Store.
Previously, I would sign the compiled app with my "3rd Party Mac Developer Application" certificate, and then in the productbuild command it would create a .pkg and sign it with my "3rd Party Mac Developer Installer" certificate.
Short answer:
A bit of a background: in Xcode 11, two new certificate types ("Apple Development" and "Apple Distribution") were added to encourage the application development of platform-agnostic applications for the Apple's own ARM platform.
Previously, developers of platform-agnostic applications might've been confused when they had to choose between the "iOS Distribution" and "Mac Distribution" certificate types, since an ARM application runs just fine both on iOS-based devices and M1-based Macs. The new certificate types remove that ambiguity.
Additionally, within this conceptual model, the "Mac App Distribution" (colloquially known as the "3rd Party Mac Developer Application", used for codesign) and "Mac Installer Distribution" (or "3rd Party Mac Developer Installer", used for productbuild) are now one certificate type "Apple Distribution", i.e. the "Apple Distribution" certificate is now able to do code signing.
You may still use the existing "Mac App Distribution" certificate for code signing and the "Mac Installer Distribution" certificate for product builds, but new certificates you create in Xcode 11 would use the new types.
