I don't know for sure, but my assumption would be that it currently isn't anything particularly interesting... Maybe things like sending updates to Windows 10's 'notification' drawer?
That's my impression without Tim Sweeney getting specific. He's more making sure Microsoft doesn't take over distribution and charge a 30% cut or something and do other forms of competitive hampering of other distribution channels like direct to gamers, or alternatives like Steam which MS is super jealous of.
AFAIK, there are no such features. Desktop app can use any UWP component maybe minus UI controls (but there's WPF)