A liquidity sweep is a candle whose wick went beyond a level while the close came back inside. The level was touched but not broken: on closes it is still intact.
It is decided the same way as a break of structure, on the same event. The count starts from the candle whose wick first pierces the level. Price stays beyond the level at the close — a break. It comes back inside — a sweep.
One move, one verdict
A pierce is either a break or a sweep, never both and never neither. In manual markup most arguments start exactly here: one person calls a pierce a false break, another calls it a break, and both appeal to "context". A close-based criterion ends the argument.
A useful corollary: churn around a round number is worth treating as one event rather than a series. Otherwise several consecutive pokes beyond one level produce three marks for the same thing.
Depth of the pierce
A sweep has a depth — how far the wick went beyond the level. It is worth looking at separately: a hair-thin touch and a full push through are mechanically the same event and entirely different in meaning.
Depth is best measured against the instrument's usual range rather than in points: the same figure means different things for a calm pair and for a volatile coin.
Which levels get swept
Sweeps are not marked only at structure extremes, although that is where it starts. Reference highs and lows of both degrees — internal and swing — are included, plus the extremes of the previous day, week and month.
Higher-period levels are exact facts rather than markup: yesterday's high exists regardless of what you count as an extreme. That is precisely what makes them convenient — there is nothing to argue about.
Why it is not a signal
A sweep states a fact that already happened: the level was pierced and not closed beyond. It does not mean price will now turn around — charts are full of sweeps after which the move calmly continued the same way and the level broke on the second attempt.
On the Smart Money page sweeps exist mainly as markup: they show where a level held, and together with breaks they describe what has already happened to structure.