I'm experienced in programming, and now I'm making a program to visualize models stored in DXF data format. However, I have difficulty to interpret the "rotation angle" attribute of "INSERT" tags.     I find that:     1) when I copy and rotate some part of the model around the Z axis, the "rotation angle" attribute is exact the rotation angle in XY plane.     2) yet, when I copy and rotate some part of the model around the Y axis, the "rotation angle" attribute is either 90 or 270.      So I'm puzzled what's the "rotation angle" attribute stands for? and how to interpret it?