The Break button breaks an entity creating a gap in it.
Quick Access Panel: 
Ribbon: Editor -> Instruments -> Break
Menu: Tools -> Modify -> Break
To create a gap in an entity:
1. Select the needed entity
2. Press the Break button.
3. Click to set the first and the second points of the gap.
Example - break a circle:
•press the Break button;
•select a circle:
•click to set the first break point on the circle:
•click to select the second break point on the circle:
•the result is:
Note: For circle and ellipse entities Break operation cuts off a sector between the two points in the clockwise direction.
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