UF_CURVE_arc_p_t的意思？

hustszh

各位大虾：
能否给小弟解释一下函数UF_CURVE_ask_arc_data中的参数UF_CURVE_arc_p_t的意思？
以下是它在文档中的英文解释，但一个晚上下来，我显然还不懂得它说的是什么意思，就麻烦一下各位了！先谢了！
The matrix tag defines the orientation of the arc. The arc center is
with respect to the origin (0,0,0) of the absolute coordinate system
using the orientation of the arc's matrix. If you have arc center
coordinates with respect to the absolute coordinate system you must
map the coordinates to the arc's CSYS and enter these mapped
coordinates into the arc_center[3] member field of the
UF_CURVE_arc_s structure. To obtain the mapped coordinates use
the following transformation mapping:
  
[X][T] = mapped coordinates
  
where you take the matrix multiplication of X and T. X is the vector
coordinates [x y z] that you wish to map, T is the orientation matrix
represented by matrix_tag. The results, mapped coordinates, are the
values to enter into arc_center[3]
To obtain an orientation (rotation) matrix rotated about an axis at a
specified angle use UF_MTX3_rotate_about_axis. To obtain the
product of a vector and matrix use UF_MTX3_vec_multiply. To create
the matrix and obtain its tag use UF_CSYS_create_matrix.
   
See the Matrices and Coordinate System (uf_csys.h)
for routines that you can use for getting the matrix_tag.

halm