Options for references.
The extent of latitude for Polar Stereographic projections, which is how far the projection extends outwards from the poles, in degrees latitude.
For example, you can use this to show data located on the south hemisphere, on a map with a north polar stereographic projection.
The default value is 90. This extends the polar stereographic projection up to the equator. Values larger than 90 extend the projection beyond the equator. The projection is only defined for extents in range ]0, 180[. Due to the large distortion beyond the equator, it's recommended to not use extents larger than 90.