Android : Does Google Fit work on its own or aside producer's SDK? How standardize it is?

Google announced Google Fit SDK. Now, does anyone know how standardize this SDK is? Does it work only with Google devices (mobiles, smart watches, etc.) or we can use it with other devices, like from Samsung, LG, etc.?

I am asking because Samsung already published its own SDK for Galaxy S5 which can access its sensors and connect it to Samsung smart watch.

The info on the Fit SDK are too weak to give me a total understanding of its SDK. We try to make a new fitness app and are not now sure whether to utilize this SDK or to use it aside manufacturer's SDK or to drop it in total.