I'm guessing that since all the "percentage of average sale" numbers are in this column that the PCT stands for "Percentage."

Auctioneer basically scans the AH and has 3 and 7 day averages for that product and keeps a running average of completed sales, as well as what percentage of AH posts for that product actually sold. Most of this information is even available to you by just mousing over the item. So, what you might see when you mouse over an item is that it has a 3 day average buyout cost of 5G, and a 7 day buyout cost of 5G 80S, and down there at the bottem is field called, (I think) "Confidence, which might show a number like 0.87. So at those prices, an average of 87 out of 100 would have sold. A few other tidbits that it will have is how many times that item has been seen during the past week, what it disenchants into, the disenchant value, the vendor value, etc. Lots of great info.

The screne that you're talking about is actually in the AH itself though. I think the tab is called "Appraiser" or something similar. When you highlight a item, it will pull up all of those items currently for sale, for example "Silk Cloth." You might have 20 entries for Silk Cloth. Auctioneer will take a look at the average cost of Silk Cloth and give a value to each stack of silk cloth currently up for auction and then color code it for easy recognition. If Silk Cloth is in a blue font, then the auction is bid only, with no buyout price. Green font is 90% of average price and less. (could be 89% or 95%, but I think it is 90%.) If the font is yellow then the price is 90% - 10% of average price. If it is in red then it is rediculous. (You can spot the people buying gold this way. An AH for Wool Cloth goes up for exactly 1000G. Someone buys it. Fair transation, right?) All the way over to the right is the PCT column. This will give you the exact percentage of average cost.

This is my absolute FAVORITE addon feature. If I need cloth for my tailor, just check the AH and grab everything in green. If I'm desparate, I might grab some yellows.

If you don't have Auctioneer then RUN, don't WALK, to your nearest Auctioneer download site and get it. Actually, just type WOW Auctioneer and their web site will show up at the top of the list.