PmCircle structure parameter definition( rTan , rPerp and rHelix)

/* Generalized circle structure */

typedef struct {
PmCartesian center;
PmCartesian normal;
PmCartesian rTan;
PmCartesian rPerp;
PmCartesian rHelix;
double radius;
double angle;
double spiral;

} PmCircle;

My question is that ：
1 . what means are parameter rTan , rPerp and rHelix ?
2. Are there has a figure that could show them clearly ?

There seems to be some clues here:
github.com/LinuxCNC/linuxcnc/blob/43ac97...th/_posemath.c#L1726

But it also appears that whilst rHelix is used, rTan and rPerp are not.

/* vector in plane of circle from center to start, magnitude radius */
pmCartCartSub(start, &circle->center, &circle->rTan);

My understanding is that rTan is the vector from center to start point.

/* vector in plane of circle perpendicular to rTan, magnitude radius */
pmCartCartCross(&circle->normal, &circle->rTan, &circle->rPerp);

circle->normal is normal of circle,where is its exact position in one circle plane ? circle->rPerp is cross production between circle->normal and circle->rTan vector. For linuxcnc, do we have original description for this definition by figure.

I draw a circle and add one rTan vector,but I don't know where is normal vector and rPerp vector ? who could please tell me about it ?

Out of interest, why are you wanting to understand all of this stuff?

What is your application? Or is it just curiosity?

Hi，Mr
Thank you for your warm reply.
If I don't understand it clearly it will be a black box to me and I could not optimize it in the next step.
Anyway, it is the first step to know its algorithm that is related to math.