Auction Lingo I Still Don't Understand

So I have a couple questions of things that still puzzle me about the auction.

1) Saying thank you when you bank? I don't get why someone must say thank you for a bank. Is there something I'm missing. Can someone explain this to me..

2) No banking under 10 slots? What does this mean exactly. I never understand when someone uses slots in their comment.

3) Number beside an arrow on an auction? What is that number mean exactly. (Ex. I have something up for auction and beside the time left in my auction is the number 29 with an arrow pointing down.)

1) Say thank you and let the auctioneer know if you're banking- keeping things in their auction as a bid so they free up some space for more monsters.
*edit* Forgot to mention gold, which is actually banking... It's safe in auctions from attackers, as long as the auction is worth more than the gold banked...
2 and 3 are connected:

The number beside the auction is how many more bids can be accepted. When you first create an auction you pick 10, 20, or 30 and the number decreases as you hold bids. Most auctioneers don't want people banking when that number is under 10, or 15 since those banking bids will be taking up space that legitimate bids could be using.

Hope this helps, at least this is what these lingos mean to me.