BSL is the level of the current strong high, SSL the level of the current strong low, and both are shown only while price has not touched them. In the terminology these are "buy-side" and "sell-side" liquidity: the assumption is that other people's stop orders rest above a high and below a low.
We mark only the observable part: the price a move started from, and the fact that it has not been touched since.
What strong high and strong low mean
Not just any extremes on the chart. The strong low is the low the leg that broke structure upward started from; the strong high is the high the leg that broke structure downward started from. In other words, the level the last confirmed move set off from.
The values live in the swing structure's state and update at every break. So there is always exactly one BSL and one SSL on the chart: not a collection of levels but a current state.
Why the level disappears
Once the level is found, one thing is checked: did price pierce it after that bar. If it did, no object is created at all.
The logic is simple: liquidity "rests" only until the first touch. A level price has already reached is not pending orders any more, it is a stretch already travelled. So BSL and SSL are fresh by definition; there is no touched state for them.
How they differ from other liquidity levels
The liquidity layer holds several different things, and they are worth keeping apart.
BSL and SSL — the two current swing-degree levels described above.
Equal highs and lows — pairs of extremes that match in price to within 0.1 ATR.
Trend liquidity — three or more pivots lying on one straight line within 0.3 ATR of it.
Previous day, week and month levels — exact calendar facts, with no markup in them at all.
What the markup does not claim
Marking a BSL does not mean price will go there to take stops. We do not know whether orders rest there: an exchange book does not show pending stop orders, and no chart shows them.
The level says exactly what is visible: here is the price the last confirmed move started from, and it has not been touched since.