What does the “Index Param Name” parameter actually do in the cache SOP?

The only documentation I can find is the phrase, “A stampable parameter set to the index during caching.”

It defaults to the letter i. I tried changing “Index Param Name” to ‘q’ and to “I_like_pizza”, and nothing seemed to change.

I've got a cache node with a stamped value for ‘Index’ that is working as far as I can tell.

Is this so that other nodes can stamp in the current index the cache is looking up, or something?