Splits all faces along a cut line. You can cut and delete faces or extract the faces
Mesh Tools > Cut Faces Tool > 
Cut Direction
Specifies how you want to cut faces.
- Interactive
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Displays a cutting line you can interactively position in the scene view to specify the location of the cut on the polygon mesh.
- YZ, ZX, XY plane
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Projects the cutting line along a plane and displays a manipulator for you to position the cut.
- Delete cut faces
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Deletes the portion of the surface to one side of the cutting line. When using the Interactive Cut setting, a dotted line perpendicular to the cut line appears to indicate the side that will be deleted.
- Extract cut faces
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Extracts faces to one side of the cutting line. When using the Interactive Cut Direction setting, a dotted line perpendicular to the cut line appears to indicate the side that will be extracted and offset.
- Extract offset
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Extract offset is available only when the Extract cut faces option is selected. Extract offset specifies the distance between the original surface and the cut and extracted piece. One practical use for this setting is to widen the model of a head. In this case, you would cut the model exactly down the center and use Extract Offset to position the extracted half further away, until the head seems wide enough. You would then fill in the gap to complete the head using Edit Mesh > Bridge.