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