index.on.role — Select indexterms based on role value
If non-zero,
then an index element that has a
role attribute
value will contain only those indexterm
elements with a matching role value.
If an index has no role
attribute or it is blank, then the index will contain
all indexterms in the current scope.
If 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
use the type attribute instead of role
on indexterm and index,
and set the index.on.type to a nonzero
value.