Free
The Free node is the default boundary condition. It means that there are no constraints and no loads acting on the edge. When the physics interface is added, a default Free node is added. If you look at the selections for this node, it will show all edges which do not have boundary conditions applied to all layers.
You can manually add Free nodes to override other boundary conditions. This is however seldom needed.
Shell Properties
The selection in this section only acts as a filter for the boundary selection.
If you manually add a Free node, it will override loads and constraints on all layers from the selected edges.
Location in User Interface
Context Menus
Ribbon
Physics tab with a physics interface selected: