Substitution Groups - Use 'em or lose 'em?

We are working on extensibility models for standardized business exchange
schemas. These schemas, under development by several standards organizations
make extensive use of hierarchical schemas and namespaces, some of which
include schemas developed by third parties.

When a user or user community seeks to use these schemas, and, needs to
modify them in some way (without altering the underlying standardized
schemas), substitution groups can be a powerful and explicit mechanism for
such extensions and restrictions.

Several participants in these standard schema efforts have expressed reserve
from utilizing the W3C mechanism of substitution groups due to their
experience with non-uniform support of parsers for this schema feature.

In your experience, what are the concerns or recommendations on the
incorporation of substitution groups into the naming and design rules of
standardized schemas?

Should substitution groups be relied upon as an extension mechanism?

Marty Burns
Hypertek, Inc.
P +1(301)315-9101
E burnsmarty@a...