Ok, let me try to explain.
(Hope I get it right - english is not main main language)
I once started to build a Ju-88.
As you shure know, its a two-engine plane with its main landing gear in the engine gondolas under the wings.
In
this picture you can see, how the gear doors are split into to pairs.
Due to the shape of the gondola, the doors don't open exactly to one of the main axes.
What I normaly would do is, build the gondola, and cut the gear-doors out.
So they keep the shape and fit in closed state.
What I now need to know, in wich angle to all axes the doors would open.
So I like to select the two vertices, wich build the edge on wich the doors are attached to the gondola, and get an info how this edge goes in relation to the 3 main axes.
I then could rotate the part around the x axis by n° and around the z axis by n°.
Snap both parts by vertice, and voilą everything would fit.
Hope thats clear...