u/seller, page-6

leyland,

If a buyer or seller wants to trade more than $200,000 worth of shares it can be entered as a U for undisclosed.
It appears in market depth as a /U after the disclosed number on that line.

ie 3 buyers for 92000/u at $1 means 2 buyers wanting a total of 92k and the u could want any number above 200k.
Once the order falls below $200k the U is removed and the rest of the order becomes visible.

The U is often used to frighten the natives as there is no way ofknowing whether it's an order for $200k of stock or $10000k of stock. So if you want to prop a price up to enable you to sell out you could put a U on the bid side just below the current high. That encourages people, firstly to buy the stock, and secondly to bid above your U so they get filled. You then sell to them while keeping you finger on the trigger ready to pull your U if it looks like getting hit.

Although this does happen I doubt it happens as often as people think. Most U's at or near the trade are probably genuine, those a few ticks below the action may be dodgy.
Mostly though, it's in the eye of the beholder.