Map images, via ImageryProviders, support Web Mercator and Equirectangular projections.
Supported vector formats CZML, GeoJSON, and KML require geometry coordinates in CRS urn:ogc:def:crs:OGC::CRS84 with optional height in meters above WGS84 ellipsoid. CZML also supports cartesian coordinates in ECEF or ECI reference frames.
Hope that helps.
Scott