Session-devices in the context of logind are devices bound to a seat and used by a session-controller to run the session. These session-devices consist of DRM (graphics), fbdev (graphics), or evdev (input) hardware devices. The work by David adds in all the infrastructure and allows for it to be used without VTs.
The session device work was merged into systemd with this Git commit earlier today.
More interesting on the end-user side is generic multi-session support by logind. This allows multi-session capabilities for seats without VTs while not changing things up if you still have VTs enabled. This work was less than introducing session-devices support and mostly came down to handling session switching.
As part of the work today was also adding in logind support for session controllers and other changes. These improvements should be part of the version systemd 208 release.