Windows Installer reference counting implementation on the registry

The old reference counting strategy was using the SharedDlls key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\SharedDlls

This is legacy, for many good reasons.
But where is the new reference counting strategy implemented?
The "new", in quote because it has been around for a while nowadays, Windows Installer way is implemented under a different registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData

where the key S-1-5-18 is for per applications installed machine-wide, while the eventual other SIDs are for the user applications.