That's a good question. It pops up at "strategically" important times I have noticed. For example, look at the image Davis posted. It doesn't show the 5th digit (but you can see the 5th digit on other trade tapes like iHub), but it went off immediately after the 1.2M buy at .0027. Is it a "code" for "I'm short on shares"?
Whatever it means - and I have to believe it means something after what showed on the tape yesterday and today - including the .0024's that showed up yesterday when the price was at least 2 ticks higher or both BID and ASK - retail traders CANNOT make these 5-digit trades. Only the MMs.