Trading above occurs when someone buys a stock for more than the best offer price.  Trading below occurs when someone sells a stock for less than the best bid.

These alerts typically signify a temporary condition where a stock is suddenly more volatile than normal.  Often this is caused by traders who know that the stock price is about to change quickly, so they choose the fastest execution venue rather than attempting to get the cheapest one.  Highly experienced short term traders may choose to join the action, in anticipation of a fast change in the stock price.  Longer term traders still take note of this condition because it is a leading indicator of which stocks will have interesting activity.

This signal is strongest when there are multiple events for the same stock in a short period of time.  When this happens the alerts server will group multiple events into the same alert.  The alert message will say something like "Trading above 4 times" to indicate that this alert includes 4 different prints that were higher than the best offer.  If it just says "Trading above" but doesn't say "times", then this alert only refers to a single print.

More options related to these alerts are described below.