A wall is a price level holding far more resting size than the levels around it. On the heatmap it shows up as a bright horizontal streak, because the colour is painted once per moment and the level persists across many moments. That streak is the whole point: the DOM would show you the size right now, the heatmap shows you that it has been sitting there for the last four minutes.
Large is a relative word
There is no contract count that makes an order a wall. Two hundred lots on the ES bid during the London session is ordinary. Two hundred lots on a thin overnight book at 3am is the largest thing on the screen. The only comparison that means anything is the level against the rest of the book at that moment, which is exactly what the heatmap's colour scale gives you: intensity is normalised against what else is resting, so a wall looks like a wall in either session.
This is also why chasing a fixed threshold fails. If you decide that 500 lots is your definition, you will find walls all day in a busy market and none at all in a quiet one, and the quiet one is where a single large order matters most.
Persistent versus flickering
Two levels can hold the same size and mean completely different things. The distinction is whether the size stays.
The persistent one is a resting order somebody is willing to be filled on. The flickering one is size being shown and withdrawn, which tells you about intent to influence rather than intent to trade. Neither is a signal by itself, but they are different pieces of information and reading them as the same thing is the most common heatmap mistake.
Not "how big is it" but "has it been given the chance to prove itself". A wall three points away has told you nothing. A wall that price has already traded into twice and which is still there has told you a great deal.
What a wall can actually be
When you see large resting size, you are looking at one of three things, and you usually cannot tell which until price arrives.
- A genuine defence. Somebody wants to be filled at that price and is prepared to take the whole flow that comes to them. This is the case that produces absorption, which is the next lesson.
- The visible part of something larger. Iceberg orders show a small slice and refill as it fills. The heatmap shows a level that keeps replenishing rather than depleting, which looks like a wall that will not die.
- Size that was never meant to trade. Posted to be seen, pulled before it can be hit. On the heatmap this is the flickering pattern above, or a bright streak that vanishes the instant price gets close.
Calling a level a spoof, an iceberg or real support while price is still away from it is a guess dressed as analysis. All three look identical from a distance. The market resolves the question by trading into it, and until it does, the honest position is that you do not know.
Reading a wall in context
Three things change what a level is worth, and none of them are its size.
Distance. A wall at the current price is being tested now. A wall five points away is a level somebody may cancel long before the market gets there. Recency and proximity both matter, and the heatmap shows you both at once because the horizontal axis is time.
Age. An order that has rested through several approaches is a different animal from one that appeared thirty seconds ago. The streak's length is the order's age, drawn to scale.
Refresh behaviour. Watch what happens as it takes fills. A level that depletes and stays depleted was a finite order. A level that takes fills and returns to full is being refilled deliberately, and whoever is doing that has more size than they are showing.
Use walls as locations to watch, not as entries. The trade is in what happens when price meets the level, and that event has not happened yet when you spot the wall. Marking it and waiting costs nothing; front-running it costs whatever the market decides.
Where this goes wrong
The failure mode is treating the brightest thing on the screen as the most important thing on the screen. Bright means large relative to neighbours. It does not mean informed, committed, or correct. Large orders are placed by participants who are wrong as often as anyone else, and the ones who are not wrong are frequently the ones hiding their size rather than displaying it.
The second failure mode is watching the wall and not the tape. A level is only meaningful in combination with what is arriving at it, which is why the footprint and the time and sales sit alongside the heatmap rather than being replaced by it.
A wall is a question the market has not answered yet. Absorption is the answer.