\pageindex and
\clauseindex respectively; there should be at most one of these in
a document and it should only appear in the preamble (i.e. between the
\documentstyle and the \begin{document}), otherwise you are
likely to get mixed page/clause indexing.
The default indexing type is by page number; this is because indexing programs seem to be happier with it. Clause indexing is aesthetically preferable, if your indexing program can cope.