Slice
Slice slices Polygon items into pieces using a LineString item. 
- Select the Polygon item you want to slice.

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Select Slice.
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Click on the LineString item.

The Polygon item is divided where it crosses the LineString item:

- This gives you a MultiPolygon item. Use Decompose [Edit-Alter] to access the separate Polygon items:

TIP: You can also use Slice to split a LineString item with another LineString item.