Those are open D&D games
… you basically play as many oneshots as you like with your characters
there are sometimes games with ongoing/continuing story arcs though
(Age of Worms, Sharn Investigations or last summer’s Ravnica games)
Theoretically you could use those also to offer other systems,
but dms/players at such tables would not get V.A.L.U.E. rewards for their D&D characters obviously. 
You can find our D&D houserules for these open games 
here