A "Selection Set" entity is a group of selections of data (Geometric, Parameter, PMI, ...) in a CAD model.
It's represented using the Dtk_SelectionSet class.
With the Dtk_SelectionSet class, you can retrieve the Name of the "Selection Set", and the list of selected data, defined by the Dtk_NodeConnector class.
The Dtk_NodeConnector class is used to specify a link to many types of data in a component,
for example to a Dtk_Node or a Geometrical Entity.

The set of "Selection Set" entities is accessible via the Dtk_GlobalDataSet associated to each Dtk_Component.

Here is a sample on how to use it :
Selection Set sample