Complex sentence: In general, the resulting parallel projection is oblique (the rays are not perpendicular to the image plane); but in special cases result is an orthographic projection, which in this context is called an orthogonal axonometry.
Split into simpler sentences: In general, the resulting parallel projection is oblique (the rays are not perpendicular to the image plane). Only in special cases is the result of the procedure an orthographic projection, which in this context is called an orthogonal axonometry.