Look at the inspector in the picture. There is a tab called Tag. Think of it as a way of organization. You have many objects and you want to check collision, but you don’t want to check all of them, so place unique tags to help filter out the selection.
A command inserted in a document that specifies how the document, or a portion of the document, should be formatted. Tags are used by all format specifications that store documents as text files. This includes SGML and HTML.