Projecting an arc on a surface
 

Hello foamers,
I want to understand sth about arc projection. I know that normally in the definition of an arc it's usually like this:

edges
(
arc 1 2 (0 0 1) // (0 0 1) is a point on the arc
//(1 2) are the two en nds of the arc.
);

The thing about arc projection is the syntax changes and I don't understand and I don't understand what it means.

The syntax when projecting
edges
(
arc 1 8 (1.5 -1 5) (1.5 -1 0)
);

and I have no idea why we're using two points in this case and where are these points located.

Thank you!