Cross trading allows a trader to view an instrument and trade the micro or mini of that symbol.  For example, a trader viewing the NQ symbol can place a trade at a desired price level and the trade will trade the MNQ.

To enable cross trading, right-click on the parent chart and check "Cross Trade". 

This will enable cross-trading for the two symbols only on the DOM attached to the chart. Please note, this will only activate cross-trading for the selected chart. If a new chart is opened, cross trading will not be enabled for that new chart.