The idea of this is to use/abuse ExecCondition= for a feature similar to the XDG autostart conditions we have.
The program uses the exact same syntax, and the names match up with the relevant systemd merge request (i.e. https://github.com/systemd/systemd/pull/15226).
i.e. this feature could/should simplify a switch to using systemd for XDG autostart files. It could also allow us to retain these conditions for the use with gnome-settings-daemon services.
CC: @aruiz (I guess you might be interested), @iainl
NOTE: I suspect it is unlikely that we will have any actual users before 3.40. That said, the systemd MR pretty much assumes that this exists already. And it might also be useful to add earlier for developer setups.