Re: [xml-dev] What does "optional" mean?

[David Carlisle <davidc@nag.co.uk>]

On 26/02/2012 16:16, Costello, Roger L. wrote:
> That is, what is it about some information that it is declared
> mandatory whereas another information is declared optional?

making the element optional in the schema has nothing (much) to do with
whether the information is required. If the information is a boolean
value it is purely syntactic choice whether true and false are:

<foo val="yes"/> <foo val="no"/>

<foo val="anything" <foo/>

so you can mark truth by the _presence_ of the attribute, which means
marking it optional in the schema. think <input selected=""/> in xhtml.
The lack of the attribute in such a case is not lack of information, it
is asserting the effective value is false.

Or it may just be that the same element has multiple roles <a id="gf"/>
or <a href=khgf">...</a> in xhtml, mean that (in some schema languages
href and id have to be marked as optional)

really it's like asking when would you begin an element name with "a"
and when with "b". It's a choice that has to be made and recorded in the
schema but it typically has no particular meaning or reason behind the
choice.

David