ABC is the frame of a correction: three legs, where A and C run against the trend and B is a pullback inside it. But the frame alone says almost nothing: there are several kinds of correction, and from the outside they all look like three legs.
What "ABC" leaves out
The forms differ not in the number of legs but in what those legs are made of.
A zigzag is 5-3-5: wave A divides into five subwaves, B into three, C into five.
A flat is 3-3-5: A divides into three rather than five. That is what separates it from a zigzag — not how far wave B travelled.
Hence a consequence worth stating plainly: saying "an ABC is under way" is saying almost nothing. A useful statement starts one step further down: which form, and why.
What is visible on the chart
Three turning points and, if the scale allows, the internal build of each leg. The second is the decisive part: without stepping down a degree, a zigzag cannot be told from a flat.
If the internal build cannot be resolved — too few legs, too few candles — the honest answer is "the form is not yet determined", not a guess.
Why B is the awkward leg
A and C have clear jobs: they move price against the trend. B takes it back, and nearly all the difficulties attach to it.
The depth of B sets the kind of form but not the form itself. Wave B may fall short of A's start, reach it almost exactly, or go beyond — and in all three cases it can still be a flat or something else, depending on the internal build.
A second difficulty: a badly overgrown B usually means the scale is wrong, and what was taken for a correction of one degree is part of a larger move.
Combinations: when one ABC is not enough
Sometimes a correction does not fit into three legs: one ABC is followed by a connecting leg and then another ABC. Such constructions are labelled WXY, or WXYXZ when there are three.
This is the least determinate case in markup. Nothing says in advance how many corrections will join up, and combinations have no constraining geometry — so there is nothing to predict their ending with.
Where people get confused
Treating ABC as a synonym for zigzag. A zigzag is only one of the three-legged forms.
Deciding the form from the depth of B. Depth gives the kind of flat; the form comes from the internal build.
Expecting the trend to resume right after an ABC. A correction can turn into a combination and run far longer than expected without breaking a single rule.