The elevation datum is an imaginary floor.  Fabrication parts will reference that imaginary floor when reporting their elevation.

So, if you set the datum to 50'-0" and draw the pipe at 0'-0" then the pipe would report its elevation at -50'-0".

The pipe is located 50'-0" BELOW the imaginary floor.

If you draw the pipe at 55'-0" then the pipe would report its elevation at 5'-0".