I'd also like to know the answer to this.
As of now, I can only think of watching on any device running W10 (PC, Phone, xbox).
Much like movies bought on iTunes restricts viewing of said movies on ONLY Apple hardware and PC's running iTunes.
Other services such as VUDU, Amazon, Google Play... have apps for different platforms and it's probably better to purchase or have your future 'library' with one of those services as they are platform agnostic. Apple and MS seem to have the most restrictions on which device you can watch your purchases on. My choice is VUDU, picture quality and reliabitly is top notch and their selections are up to date and priced at or less than others. I DO like the MS Movies and TV app on my w10 desktop and would love to be able to buy movies there and watch them on my roku or ps4, but can't right now. Vudu is currently available for ALL devices, and will be coming to apple tv soon. You can even download the vudu purchases for watching offline on ALL platforms.