Dear Carl,
i've reformatted your posting lightly to increase readability.
Concerning @cr_fields: The term refers to the fields of the content repository, which are part of every content repository entry ("name", "title", "description", "nls_language", and also the "creator" field, which is defined in xowiki::Page). Since form pages extend these pages with arbitrary additional attributes, these native names of the content repository are prefixed with an "_" to allow same-named fields for extending as well. So, instead of defining in the form_constraints attributes for all of these fields, one can use the short-form @cr_fields. This is just a convenience for the developer.
The meaning of "hidden" in this case is, that when one edits such a page, the edit page does not render input fields, but hidden fields. If "omit" is specified, these fields will be omitted.
if you look for the source code, see e.g. "xowiki-www-procs.tcl", where the value of this entry is passed in `-field_spec`. Typically, this value is set in user-level forms.
