Launch the Cross Section window 
from the sidebar and press Activate.
Set plane
Choose from the following modes to define cross-section planes interactively.
Horizontal defines a horizontal plane at the desired height with one click in the viewport. You can find the exact z-value in the Cross Section window. To fine-tune this horizontal plane, adjust the z-value or select Set plane > Parallel and change the height in the viewport with another click.
Vertical defines a vertical plane from two user-selected points in the viewport. To adjust this vertical plane, adjust the rotation angle 
or select Set plane > Parallel to move the vertical plane parallel in either direction.
Free lets you specify a custom plane from three points. To edit the plane, change elevation
and rotation angles 
, or select Set plane > Parallel to move the plane parallel in either direction.
Parallel sets a new plane origin while preserving its orientation. It is particularly useful to move existing plane definitions parallel in either direction.
When setting your planes interactively using Set plane, you can snap to the grid, curves, meshes, and other sections. Simply activate the desired snap modes from the toolbar, then set the plane. When changing individual plane definition parameters with the mouse wheel, use CTRL + mouse wheel or SHIFT + mouse wheel to increase the increments of each scrollCopy from section
The Copy from section dropdown lists all sections available in the scene. Select the desired section to update the cross-section plane parameters.