isolation level "indexed repeatable" ?

Hi. browsing through the "Informix Universal Server Performance Guide 9.1" I noticed a fifth isolation level. Or to be more precisely, the "repeatable read" is split up in "indexed ..." and "sequential ..." (page 2-34, LOCKS parameter). Is this another isolation level? The Syntax Guide doesn't list it. At least not for the Universal Server 9.13. Or does it depend how the operation is done? How can I turn on the "indexed repeatable read" isolation level? Chris.