So we see 2 tables, one for bids and one for asks. For each, we see 3 columns. What I don't understand is the MMID column.
I understand that it stands for Market Maker ID. But as far as I know, a market maker is a stock exchange participant. And from these 4 letter identifiers it seems that they stand for different stock exchanges, not specific stock exchange participants.
Is there some inter-stock-exchange activity? Are there securities passing from one stock exchange to another?
Would be great if someone can elaborate.