The only difference is the direction relative to the current trend. A BOS is a break through a level in the direction of the trend, confirming the move continues. A CHoCH is a break the other way, the first sign that structure is changing. The mechanics of detecting them are identical.
How a break is decided
A level counts as broken on a close, not on a touch, and the decision is made over an episode of three bars.
The count starts with the first candle whose wick pierces the level. Then we look at where the close lands by the end of the episode. If it is beyond the level, that is a break on a close, recorded at the first close beyond it. If price came back inside, the wick was a liquidity sweep, not a break.
There is one rule and one branch, so the same episode can never be counted both as a break and as a sweep. That matters more than it sounds: in manual markup this is exactly where most of the arguments start.
When it is a BOS and when a CHoCH
Each degree of structure keeps its own state: the current direction plus the reference high and low. A break is compared against that direction.
The trend was up and the previous high broke — that is a BOS, a continuation. The trend was up and the previous low broke — that is a CHoCH, a change of character. In a downtrend it mirrors.
It follows that the same break can be called different things at different degrees: for the internal structure it is already a CHoCH, while for the higher degree nothing has happened yet.
How it differs from MSS
An MSS is the same break, but made by an impulse. There are two conditions and both are required: the leg, measured from its start to the break bar, must span at least one and a half ATR, and a same-direction gap must fit entirely inside it. Span alone is not enough — without a gap the price moved tightly rather than impulsively.
So MSS is not a third kind of break, but a BOS or CHoCH with an extra property.
What a break does not mean
A break of structure is a statement of fact: the level was broken on a close. It does not mean the move will continue, and it is not a signal. On the chart it is needed as the support for everything else: an order block is looked for precisely before the impulse that made the break, and without a break there is no block.