Thank you for the follow up info. I had missed the part of the documentation where X is 1,2,3... I see now that item_cart_position describes that X is for the other variables too.
Overall I find the documentation a little quirky; information that could exist in a single place is spread out over 3 or more places (like post variables), and following the above example, things are not always very clear (would love to see more template examples and of course MySQL data types; such info would help E-Junkie grow, I think).
That said, it's still a wonderful service! Thank you!