Fizzlemizz, I left this thread open on my phone's browser just after your post, so I didn't catch your edits either lol.
Anyways, to expand on InventorySlotId:
https://wowpedia.fandom.com/wiki/InventorySlotId
And PutItemInBag:
https://wowpedia.fandom.com/wiki/API_PutItemInBag
Basically, your character's entire "inventory" is actually your equipped items, your equipped bags (where items are saved elsewhere), your 28 base bank slots, and then your equipped bank bags. On retail, this counts all the way to 86. 0 is the old unused ammo slot, 1 is your head, 2 is your neck, 19 is your tabard, 20 is the first bag next to your backback, 21-23 is the next three. I'm honestly not sure what 24-51 is, then 52-79 is your base bank, and finally 80-86 is your bank bags.
Also, Fizzlemizz nailed the table explanation. It's a temporary table that starts off blank every time you use this code and is a quick solution to track slots where your unequipped items get dropped into. The slots may actually not line up depending on how you have new items enter your bags, but it still works in the end because the loop will find the, say for example, four empty slots and the table will treat those spots filled in the eyes of this loop when it dumps four of your pieces in.