You can make smooth patches from rough polygons automatically by setting a patch in the property of the object panel. There are four types of a patch in this version.
Kind of Patches
Spline Type 1/2
When you use [Spline Type 1] or [Spline Type 2], polygons must be quadrangles only as a general rule. A triangle can be mixed, but it must be surrounded by three quadrangles. A hole opens if the neighboring face is not a quadrangle or there is no face next to it. As two triangles can be connected each other, you can combine them into one quadrangle.
The center triangle is converted to a patch because it is around by quadrangles.
A hole opens because two triangles adjoin.
Catmull-Clark / OpenSubdiv
[Catmull-Clark] can make patches from triangles, quadrangles and polygons. It becomes quite rounded compared to the former 2 types.
Triangles, quadrangles and polygons are subdivided by Catmull-Clark or OpenSubdiv patch.
Freeze
When you want to convert patches to polygons, select [Menu > Object > Freeze]. Patches are mostly frozen automatically when you save a file except MQO format.