In TOS, BS usually means bid size and AS means ask size. They show how much size is sitting on each side of the market at a given price level.

What they mean

  • BS = Bid Size : the number of shares/contracts currently bidding.
  • AS = Ask Size : the number of shares/contracts currently offered for sale.
  • In Level II or order-book style displays, these values help show market depth and queue size.

Why it matters

  • A larger bid size can suggest stronger buying interest.
  • A larger ask size can suggest stronger selling pressure.
  • The numbers can change quickly, so they’re best read as a snapshot, not a guarantee of direction.

Simple example

If you see BS 500 and AS 800 , that means there are 500 units on the bid side and 800 on the ask side at that moment.

TL;DR: In TOS, BS = bid size and AS = ask size.