There was a time when Index Shifting didn’t exsist, you shifted gears by just moving the shift lever until it started to shift then move it some more until it stopped making noises and was in the next gear that you may or may not wanted. in about 1986 or so Shimano introduced an Index shifting system that you moved the lever one click and it went to that gear, no fuss no muss, they termed it SIS for “Shimano Indexed Shifting” the upper range grouppo Shimano 600 was the first one to be released to the masses. It was already on Dura Ace but at the pro level with alot of limitations on gearing. later to be used on the whole Shimano range including the new grouppo 105. the SIS became the standard for all derailleur systems. Suntour and Campagnolo later followed with Suntour Accushift and Campgnolo Synchro.