Start by aligning your 2D plan the way you want by using the Rotate tool.

Next choose the Section: Create clipping planes and drag a rectangle to indicate the vertical clipping planes.

Select the height of the current storey/floor or the full height. Click Clip region.

You can now move the clipping planes as normal by selecting the relevant plane in the clipping widget (bottom left), or by pressing "Shift" and clicking on the plane in the viewer. Planes can be removed individually by selecting a plane and pressing "Del" or all at once by right clicking anywhere in the 3D viewer and selecting "Remove clipping planes"

Section: Select objects will work in a similar way and will select anything touched by the "virtual box" defined by the selection rectangle and height. To get back to normal selection in 2D click Select.