A geometrical domain can be described depending on its highest dimension by volumes (3D-entities), faces (2D-entities), edges (1D-entities) and nodes (0D-entities). You can use those entities in the simulation design in order to assign materials, apply boundary conditions etc. Often, it is convenient to group some of these entities and access them as a whole e.g. for a boundary condition assignment. Such a group is called Topological entity set.
In order to create a Topological entity set, you have to select the entities that should be grouped in the Viewer at first. If you want to create for example a face set (e.g. group multiple inlet faces for an internal fluid flow simulation), enable the Pick faces button in the Viewer and click on all faces that you want to add to your set. When all faces of the group are selected (i.e. highlighted with red color) click on the Create Set button in the Settings panel and choose a name for the set. You may now access the new set while specifying the simulation setup or perform some simple actions on it in the panel.