This compares the last print for this stock to the best bid and offer.  If you set the max to 0, you will only see stocks which were trading at or between the bid or offer at the time of the alert.  If you set the max to 0.1 you will only see stocks which were trading no more than one tenth of one percent above the offer and no more than one tenth of one percent below the bid. 

This separates legitimate prints from bad prints.  The further the print was from the inside market, the less reliable the alert is.  This is especially helpful when used with the OddsMaker.  The OddsMaker uses the price of the last print as the entry price for the trade.  This is only useful if this was a print that you could actually get.  The closer the print was to the inside market, the more this is as an entry price.