The polygon_filter parameter restricts the number of individual polygons returned by the Isochrone API. When enabled, the API returns at most the number of polygons specified in the parameter value.

It does not combine polygons together, but simply selects individual polygons from the returned shape. Where there are more than the number specified, the polygons returned are the largest ones by area.

This parameter is available through both the /time-map and /time-map/fast endpoints.