Include data that may be objectively generated some day?

Hi Folks,

Is it good or bad to include, in an XML document, data that is currently not objectively generated but may be in the future?

Please allow me to explain.

Example: Below is information about a book. Suppose that today's applications effectively process its markup and data:

<Book>
      <Title>_________</Title>
      <Author>_________</Author>
      <Date>_________</Date>
      <ISBN>_________</ISBN>
      <Publisher>_________</Publisher>
</Book>

Now, consider the problem of determining how "enlightening" a book is. Assume that there are no objective rules for determining the enlightenment-capability of a book. Manual determination of the enlightenment-capability of a book is subjective and yields mixed results. Thus, automating the determination of the enlightenment-capability of a book is problematic.

Nonetheless, the developers of the Book XML vocabulary decide that someday it may be possible to objectively measure the enlightenment-capability of a book and applications will make decisions based on enlightenment-capability data. So they decide (today) to require that each book contain an <Enlightenment-Capability> element:

<Book>
      <Enlightenment-Capability>__________</Enlightenment-Capability>
      <Title>_________</Title>
      <Author>_________</Author>
      <Date>_________</Date>
      <ISBN>_________</ISBN>
      <Publisher>_________</Publisher>
</Book>

Since the XML documents contain enlightenment-capability data application developers figure "Hey, let's use it."  So, suppose applications (today) start making decisions based on the data in the  <Enlightenment-Capability> element. What will be the consequence?

More generally, what will be the consequence of adding this element to the Book XML vocabulary?

/Roger