Meaning of block="substitution"?

Hi Folks,

This question is with respect to block="substitution".

Example.  Consider this head element declaration:

<element name="subway" type="string" block="substitution"/>

Does the block="substitution" mean that it is invalid to even declare a
substitutionGroup element, such as:

<element name="train" substitutionGroup="a:subway" type="string"/>

Or, does it simply mean that in an instance document <subway> cannot be
substituted by train?  That is, does it mean that it is valid to declare
substitutionGroup elements but it is invalid to use them in instance
documents?

/Roger