Whats the difference between Surface vs Open PolySurface

Hi,
I would like to know what the differenece between an open polysurface and a regular surface is. is there any advantage or disadvantage between them.

In the photo the surface in blue is the open polysurface and the purple one is just a regular surface. Both surfaces were made from curves using the EdgeSrf command, why does it sometimes generate two different surface types.

thanks

Hello - a polysurface is two or more surfaces joined at the edges. If there are no unjoined edges, then the thing is a closed polysurface.
EdgeSrf, like Loft, pays attention to the structure of the input curves - if these have kinks, or hard corners or fullt multiple knots then the output is likely to be split into multiple faces (a polysurface) at those locations.

-Pascal