A Tag is another name for an attribute. Take a look at these examples. Note that sometimes attributes contain further data. Subject for example. That data may be free-from or, in the case of things like dates it may have a set format. Sometimes there is an overlap between the data content of the attribute and other attributes: Is 'Orchid' an attribute of the photo or is the the data that is associated with the Subject? It is both of course
But why should I not be able to associate all of these 'tags' with my files. And why should I not be able to use any, all, or a subset of them to identify the file I want to use?