The first limit is structural. A return has to be welded to a face along its whole length, and below a certain stroke width there is not enough face to weld to without burning through. This is the limit that stops a letter being fabricated at all.

The second is distortion. Welding puts heat into thin material and thin material moves. A stroke can be wide enough to weld and still come out of the jig bowed, which means it will not sit flat against a wall.

The third applies only to illuminated letters: an LED and its wiring have to fit inside the return. That threshold is wider than the other two, which is why a design can often be built in solid stainless and not in face-lit LED at the same stroke width.

When we look at a drawing we say which of the three it crosses. A design that fails the third can often be built as a halo-lit or solid letter instead of being redrawn.