Primary indexes and index-sequential files

I am studying the physical organization of databases and right now I trying to understand the concept of primary index or clustering index. The book states the primary index can be realized by storing the tuples on the index itself ( the index stores the tuples ). According to the book, in this case (the index is not separated by the file containing the tuples) the primary index is so called because the storing method can be done by storing according to the search key but also because the index contains the physical storage necessary to store the tuples. The book states also that this kind of index can be also realized by constraining tuples to stay in a certain position according the search key (so they can not be moved from there). So the questions are: 1) How many ways are used to realize a primary index not separated from the file containing the tuples? 2) When can we call a primary index also index-sequential file? Thank you