Hi,
I’d like to export the Cartesian coordinates of the centerline which is extracted by extract centerline mode. What does R,A,S stands for? Is they correspond to x, y, z in Cartesian coordinate?
Right, anterior, superior. These are anatomical coordinates, see Coordinate systems - Slicer Wiki
Graham Wideman also has a relevant page on that at his website:
http://www.grahamwideman.com/gw/brain/orientation/orientterms.htm