Rear derailleur, what does top normal mean?

I need a new rear derailleur. The current one is a mountain bike part, an Acera 7/8 speed, but it's on my road bike because of the megarange cassette.

Perusing Ebay, the similar derailleurs are often described as "top normal to suit rapidfire changers".

Is this compatible with STi changers? What does "top normal" actually mean.
 
From wiki:

High normal or top normal rear derailleurs return the chain to the smallest sprocket on the cassette when no cable tension is applied.[9] This is the regular pattern used on most Shimano mountain, all Shimano road, and all SRAM and Campagnolo derailleurs. In this condition, spring pressure takes care of the easier change to smaller sprockets. In road racing the swiftest gear changes are required on the sprints to the finish line, hence high-normal types, which allow a quick change to a higher gear, remain the preference.
Low normal or rapid rise rear derailleurs return the chain to the largest sprocket on the cassette when no cable tension is applied. This shifting system was developed by Shimano in 2004 and can be found on their XT and XTR mountain derailleurs.[10]
In mountain biking and off-road cycling, the most critical gear changes occur on uphill sections, where riders must cope with obstacles and difficult turns while pedaling under heavy load. This derailleur type provides an advantage over high normal derailleurs because gear changes to lower gears occur the direction of the loaded spring, making these shifts easier during high load pedaling.
 
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