So complicated, so confusing, and in effect absolutely **** useless
(since ATC will probably never send you that way) which is why I never used the 'extend the centreline' (luddite!) but always knew where I was and what I height I needed to be. Much simpler. Is it that difficult?
Unfortunately it became the 'darling' 'clever trick to show' for those TC's who presumably did not really know where they were and every F/O came out of the sausage machine programmed to push the buttons and then not really know what to do with it.
The very best in FMC vertical guidance was with an un-executed (dotted white on 737) leg to a point, but that sadly fell out of favour with de management and became a no-no..
Long live range to waypoint and the three times table. Why complicate it?