Type: enumerated , one of ( "geojson" | "vector" | "raster" | "image" )
Also note:
source
Parent: layout.mapbox.layers[]
Type: number or categorical coordinate string
Sets the source data for this layer (mapbox.layer.source). When `sourcetype` is set to "geojson",
`source` can be a URL to a GeoJSON or a GeoJSON object. When `sourcetype` is set to "vector"
or "raster", `source` can be a URL or an array of tile URLs. When `sourcetype` is set to "image",
`source` can be a URL to an image.