You can lock pipe elevation as you draw a horizontal pipe run.
By restricting the pipe run to a specific elevation, you can avoid unintentional snapping to another object at a different elevation. When you want to connect a horizontal run to a vertical segment, locking the elevation prevents the horizontal segment from snapping to one end of the vertical segment. It allows you to connect along the segment at the desired elevation. You can turn on the elevation lock for a portion of a pipe run, and turn it off for other portions where you want to use different elevations. If the lock is turned on, you cannot connect to an object on a different elevation.
Consider a scenario where you are working in the top view. You attempt to connect a horizontal run to a riser. You configure 2 viewports in the drawing window. The top view is used in the left pane and an isometric view to the right to preview routing solutions. In the top view, only the pipe end connector is available on the riser.
If you select the pipe end connector, routing solutions are offered that do not connect the horizontal run to the riser at the correct elevation.
If you lock the elevation before selecting the pipe snap on the riser, you connect to the riser at the elevation of the horizontal run.
You can lock the elevation for a portion of a pipe run and unlock it for other portions where you want to use different elevations. If locked, you cannot connect to an object on a different elevation. Additionally, you cannot lock elevation on a sloped pipe segment.