pretty much from skimming through the book Blackjack Attack, i believe I18 was 'framed' around a four deck game, probably S17 (my guess). my take on the I18 is that it can pretty much be used for 1, 2, 4, 6 and 8 deck games. probably it would be best for 4 deck games, but still useful for the others. it seems it was meant to be a bit of a trade off, a one shoe fits all (but 4deck best) sort of heuristic approach. with so many game rules, # of decks, it's well, there are loads (hundreds) of index departures one might be confronted with memorizing and categorizing, sorta thing. the I18 gets around that, at relatively little cost, is the idea.
far as H17 vs S17, there may be a difference, i'm not sure, but probably not a radical cost, would be my guess. please don't take my take on it as the final word. Don's around and hopefully he'll illuminate your question.