What we opened
Two commercial 55-inch panels: one edge-lit, one direct-lit. Both rated 500 nits, both 3.5 mm combined bezel. The edge-lit unit is 42 mm deep. The direct-lit unit is 51 mm deep. That 9 mm is the whole story.
The edge-lit stack
A strip of LEDs runs along two or four edges of the panel. A light guide plate spreads that light across the back. A diffuser smooths it. The advantage is depth and cost. The disadvantage is that uniformity depends entirely on the light guide, and a light guide is a precision optical part that does not like heat cycles.
The direct-lit stack
LEDs are arranged in a grid behind the panel, usually 12 to 24 zones in a 55-inch commercial unit. No light guide. The diffuser is thicker. Uniformity is easier to achieve and easier to keep over time. The cost is depth, weight, and slightly more power.
Where the difference shows on a wall
Put both panels side by side showing a full white slide. The edge-lit unit will usually read 3 to 6 percent dimmer at the corners. On a single screen nobody notices. In a 3×3 wall, a corner that is dimmer than its neighbour becomes a visible seam of a different kind.
Where edge-lit is the right choice
Menu boards, elevator displays, narrow totems, and any installation where depth is tight. A 42 mm panel fits in a 60 mm wall cavity. A 51 mm panel does not. If the wall is 100 mm deep, edge-lit is not a compromise, it is the only option.
What to ask for
Ask for a 9-point uniformity measurement on the actual unit, not a series average. Ask for the depth of the panel including the bracket. And ask whether the light guide is replaceable, because on a long-life installation that part, not the LED, is usually what fails first.