RE: multi auto trades at 8.0531 Jan 2018 19:28
Just seeing the front of the BID ASK is not useful either, you need to see the L2 book to see depth.
In non sets you only see the front of the BID and ASK, nothing more. However...
In SETS you see all the 'market buyers (BID) and market sellers (ASK)' the price and the amount they have for sale or are bidding for.
Lets just look at the BID -
So on BID at 37p there could be one Buyer (bidder for your shares you wish to sell) looking 1000 shares. This is the front of the BID. When someone sells at market they will fill that buyers bid of 1000 shares at 37p
But below this 37p could be two buyers on 36.90p looking 20,000 shares, 3 on 36.75p looking 30,000 and then a gap to 36p a buyer looking 100,000. First person to put order in at a price is first in line at that price.
Buyers on BID could be SETS market makers, but rarely at the front of the spread. The other buyers could be dealers/banks....even YOU! (if you have paid for DMA/L2 access). A person on the BID looking to buy could actually be a short looking to close their short by buying. When a market price is paid you see AT trade and the matched trade on the book dissappears and that is when you see a price movement if the shares at that price are all taken. The next price with shares being wanted is next. In the case above if the 37p is taken out the BID will now show 36.90p. O trades are negotiated trades often inside the spread between dealers and you, only time outside spread is if its big. But at any time buyers on the BID could 'pull' their order and the price drops to the next price etc. So when market moves people will pull their orders to put them back in further down. Going back to split trades at exact same time.
BID looks like this
37p 500 shares
36.90p 5,000
36.90p 15,000 (but you would see 2 36.90p 20,000 perhaps)
36.75p 10,000
If someone in a hurry says sell now at market 30,400 shares you will see on the trades
37p 500 AT
36.90p 5,000 AT
36.90p 15,000 AT
36.75p 9,900 AT
You don't see 30,400, you only see 30,400 if there were 100,000 shares at one price bid by one person and the 30,400 is taken in full.
Above you will now have left
36.75p 100 shares at the front of the BID and that person's bid still 100 shares short of being filled. So next market trade for 1000 shares, will take that 100 and then move to the next offering at 36.75p, so hence why you will see small amounts sometimes (but often at same time as other AT trades).
You could just get a trade for 59 shares say, but its single if it has its own time, when it is beside 3 otrher trades its just a partial fill of a BID order.
See an example of an L2 offering here which is IG's L2 dealing platform - http://www.l2dealer.com/content/files/l2_manual_lowres.pdf - go to page 9 of 31.
This is Vodafone so the book is way thicker than that for say VOG. It has 11 people at the front of the BID for a total order of 435k shares.