Basically, the Selection Object is an object that will save selections in your object manager. For instance, I currently have a project that I need to keep adjusting groups of plain effectors, but are children of multiple objects scattered throughout my scene. I’m able to select them all at the same time, with this object. Great for dense scenes.
I have no idea where it is in the menus or if it is even a default menu item. Do a search for Selection in your customize window. You can add that tool to a palette.