While Map Mavin recognizes a wide variety of reference systems and users can manually set it to recognize even more, we store all of our spatial data in Web Mercator WGS84. You can find more about the spatial reference system we use here.
So what this means for you is that all data uploaded to Map Mavin will be saved in this Web Mercator WGS84 reference system. And when you download your datasets, you will see them in this new spatial reference system. This also includes any vectors files you edit, as your edits will be saved to a file in Web Mercator WGS84.