The parallel snap is used draw lines parallel with the edges of existing objects. The parallel snap check box is displayed under 'Direction snaps' in the 'Drawing Options' data panel when the various drawing commands are active. To draw a block perimeter line to be parallel with an existing block edge, switch the parallel snap on and move the mouse cursor to touch the edge with which the current perimeter line is to be parallel:
This will enable the active perimeter line to infer a direction from the selected edge and you can lock this inference by pressing the SHIFT key to prevent a parallel inference being picked up from an unwanted edge. So press the SHIFT key and move the mouse cursor to a position approximately parallel with the edge, the cursor will 'stick' when the parallel snap operates and the perimeter line will change colour to cyan:
When the parallel snap is in operation, intersections are automatically calculated with drawing guides to allow the two to be combined, so you can snap to the intersection of the two.