The TPO summary shows values and calculations for the current TPO (letter at a specific price.) The current TPO is whichever letter and price last traded. However if the crosshairs are enabled on the chart, the current TPO tracks whatever TPO is under the mouse as it is moved over the profile.
TPOThe current TPO.
VOLThe total volume traded for this TPO.
DThe difference between the volume traded at the best bid and the volume traded at the best offer aggregated for this TPO.
BADisplays the volume traded at the best bid and offer.