I always use the FMC. As I noticed it controls only decents and only when I pushed the NAV-Button on the FMC. The Autopilot the decents to the altitude witch i have Input at the waypoint in the FMC.
Oh sorry, my english, I hope you understand me.
When i loaded a flightplan, or when i enter the waypoints by my self, the I DON'T push the NAV-button on the FMC.
The the autopilot climb and decents (when I push AltSel) always to the altitude what is selected with the Altitudeselector. No matte what altitudes are in the FMC.
So i think, the FMC only serves ist altitudes to the Autopilot when the NAV-button on the FMC is used.