I was initially confused about the standard coordinate system and the use of reference geometry. It turns out to be very useful in certain cases such as displaying displacements or vectors relative to a specific plane (or viewing perspective) or applying displacement components in a fixture. -- John Willett

Dongwei -- Perhaps some of the (rather convoluted) discussion in the following thread will help you:

Definitions of the "Standard Planes"?

I was initially confused about the standard coordinate system and the use of reference geometry. It turns out to be very useful in certain cases such as displaying displacements or vectors relative to a specific plane (or viewing perspective) or applying displacement components in a fixture. -- John Willett