Node of the TShapeTree representing an alternative, choosing one (or none) child from it's children list as active.
It's ideal for representing the VRML >= 2.0 Switch node (not possible for VRML 1.0 Switch node, as it may affect also other nodes after Switch). Actually, it even has a SwitchNode link that is used to decide which child to choose (using SwitchNode.FdWhichChoice).