index.on.type — Select indexterms based on
index element that has a
value will contain only those
elements with a matching
type attribute value.
index has no
attribute or it is blank, then the index will contain
indexterms in the current scope.
index.on.type is zero, then the
type attribute has no effect
on selecting indexterms for an index.
For those using DocBook version 4.2 or earlier,
type attribute is not available
for index terms. However, you can achieve the same
effect by using the
in the same manner on
index, and setting the stylesheet parameter
index.on.role to a nonzero value.