index.on.role — Select indexterms based on
index element that has a
value will contain only those
elements with a matching role value.
index has no
attribute or it is blank, then the index will contain
indexterms in the current scope.
index.on.role is zero, then the
role attribute has no effect
on selecting indexterms for an index.
If you are using DocBook version 4.3 or later, you should
type attribute instead of
and set the
index.on.type to a nonzero