How works Scale in V3d_View ?

I have to create a photo realistic image of a “3d_view” (OCC) with a ray tracing engine.

I have already get objects from the scene, and I would like to understand better how the position's camera works in OCC. (perspective model).

Eye and At define the direction in which the camera look at, right ?

Up indicates the direction of the top of the “image plane” (the view)

It seems that Focale is the distance between FocalReferencePoint and the “projection plane”, but I'm not sure...

I understand that a shorter Scale gives a wider view while a longer Scale zooms in for close-ups.

But what is Scale based on ?
I mean, is it a ratio between two “items”( FocalReferencePoint, Eye, At... ) of the camera or other thing ?

Thank you in advance.

Best Regards

Lionel

PS : The raytracing engine I use doesn't have “Scale”. That's why I'm trying to re-interpret it.