Regarding the "tags" that can be applied to a file with any given extension:
The technical term is "property", rather than "tag", and that's not to pick a nit, so to speak, but if you go searching Microsoft's MSDN site or performing more general Google searches, referring to file "properties" and the "property store" will net you much more useful results w/respect to the search terms that are used. With that said about the semantics, what is important to know is that the Windows Property Store functionality is, for the most part, implemented almost entirely by Microsoft, although it can be extended with 3rd Party Property Store Providers that can be registered to manage the properties that can be applied to files with certain file types. With the functionality present in the default property store provider, Microsoft provides for a finite set of properties for each file type/extension that is supported. You can't add arbitrary properties/tags to just any old file type.
PSExplorer.wbt is written to present you with the possible set of property names that are supported for a file of any given type/extension. If a specific property is absent, then the text box for the property value is disabled. If the property is present, then the text box is enabled and the value is present, including a value that is an empty string of zero length, which is different from a value being absent entirely. Depending on what types of applications have been installed on the system, there may be additional properties available for additional file types/extensions.
The Golden Rule of the Windows Property Store is that if you can view/edit the value on the "Details" page of the file/folder Properties Panel in the Windows Explorer, then the Property Store Extender can access it as both are using the same underlying Windows-provided facility to access the properties.
Note... simply renaming a file to have a particular file type/extension doesn't imply that you can hack your way into adding properties that are otherwise inappropriate for the file's original type/extension. The Property Store Providers are implemented as DLLs that contain code that can parse the binary format of various file types and properly manipulate their supported properties. If you were to rename a file from ".doc" to ".mp3", it wouldn't allow you to add MP3 IDv3 tags to a Word document. Instead, what you'd get is some kind of error, with the Windows Explorer not showing any defined properties beyond the basic file system attributes, time stamps, size, etc..., and with the PS Extender most likely returning some kind of error when attempting to load the properties for the file that's masquerading as a MP3 file.