Well-established uses of processing instructions?

[Vladimir Gapeyev <vgapeyev@seas.upenn.edu>]

I am curious to know to what uses have people put XML processing
instructions.  I am more interested in those that have a "community"
around them (manifesting itself by a de jure or a de facto standard, or
multiple applications that understand the PI, or large amount of document
instances with unrelated authorship) rather than in made-up samples,
however plausible they are.  Any pointers?

After some search, I am aware of just a couple instances:

(1) http://www.w3.org/TR/xml-stylesheet/ -- W3C Recommendation of 1999 on
Associating Style Sheets with XML documents.  How much is this Rec
important for non-HTML XML documents?  (For (X)HTML, the <LINK
REL="stylesheet"> element appears to be a better solution.)

(2) It appears Apache project Cocoon-1 used PIs, but Cocoon-2 switched to
a more flexible model where association between an XML document and a
processor is specified by a third, separate, document
(http://wiki.apache.org/cocoon/ThePyramidModel?highlight=%28processing%29%7C%28instruction%29).

Thanks for any help,

Vladimir