Schema development for information design & authoring

Thank you for the answers that confirmed my understanding.

I didn't mean to omit XSD (it's actually the one I know more), but with
reference to technical documentation, the main two current applications
use DTDs (DITA, DocBook to 4.5) and NG/Schematron (DocBook 5.0).

I guess for authoring purposes, in most cases industry standard schemas
will suffice. Especially with DITA specialization feature. Probably, not
in too many instances that a business will choose to develop their own
schema (generically speaking, incl. DTD).

From a schema development perspective within this scope, it seems to me
that it is more of concern to broad info architectures and tool
developers, and may be content management. But one motive I see for
developing own in narratives or paginated documents, is that
customization and/or parameterization of standard ones isn't very
straight forward. Developing own schema that is more requirement
specific simplifies the development of scratch transformations to assume
better control over publishing appearance -- unless investment in
high-end tools.

This is really the niche where I was interested in finding more about
how likely/often a practitioner would need develop custom schema.

Any further comments appreciated.

Regards.
Khaled