Thanks for answers.
If I finally came to understand the difference between these coordinate systems, then please check me:

if several (or many) points lie on the same level surface (for example, at sea level), then

1) then they will necessarily have a different "Z" coordinate in jag3d, provided that "local elipsoidal system" is selected;

2) then they will necessarily have almost the same "Z" coordinate in jag3d, provided that "local Cartesian system + Earth curvature" is selected.