SDS

The Sub Division Surface is a method that transforms a raw mesh into a smooth one. The user works on a raw mesh (or cage) and ppModeler computes interactively the smooth mesh and displays it.

A mesh without SDS activated.

The same mesh with SDS activated.
Any mesh can be use as a SDS cage, even symmetrical mesh. There are two meshes displayed for an SDS mesh: The raw mesh (or The 'Effect' menu holds the commands to work on SDS mesh: The short key [CTRL-S] can be use to convert back and forth from mesh to SDS. The cage is always displayed in wireframe, whatever is the current drawing mode. The SDS mesh is displayed accordingly to the current drawing mode. The level of subdivision changes the smoothness of the SDS mesh at the cost of complexity:

SDS mesh at level 0 (raw mesh).

The same mesh at level 1.

The same mesh at level 2.
 
When one SDS mesh is selected, the short keys 0, 1 and 2 can be use to change the level of subdivision to the corresponding level. The level 0 and 1 are useful to edit the mesh. Level 2 is quickly (in term of number of faces) to slow to update SDS mesh in real time, but can be used from time to time to have a little preview.
An SDS mesh can have 2 types of edges: smooth and sharp. The edge's type can be changed in the parameter area, when some edges are selected:

Sharp edges separate the mesh in two parts smoothed independently. When one edge is set to sharp, nothing happens. But with 2 connected edges, a sharp line appears on the SDS mesh:

The 8 selected edges are set to smooth.

The same 8 edges set to sharp.