These alerts will not appear every time the market is crossed.  Crosses often appear in groups.  The alerts server will filter these, and report the first crossing in each group.  It will report new alerts only if the size of the cross grows, or if the market has been uncrossed for several minutes before crossing again.  Some stocks, particularly the highest volume stocks, are crossed on a regular basis.  The alerts server may filter out most or all of the alerts for these stocks.

In some cases the alert server will describe the alert as "up" or "down".  This distinction is based on the primary market.  The assumption is that the primary market does not react as quickly as the ECNs.  So if the bid on an ECN is higher than the specialist's offer on an NYSE stock, many traders assume the price will move up soon.

Note:  These alerts are only intended to highlight stocks which are doing interesting things.  A crossed market is often a leading indicator of other activities.  These are not intended for arbitrage.  Crossed markets typically last for only a second or two, and disappear before most traders can take advantage of them.

More options related to these alerts are listed below.