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 ?

/* 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 ?

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.

Hello,everyone

Who could please give me some hints for this topic ? So far rTan , rPerp and rHelix parameter definition is obscured for me.

Hello,Mr
Firstly, thank you for your warm reply for this quesiton, like what you said in your previous answer,it is difficult to understand how rTan , rPerp and rHelix work in linuxcnc,I just use it as black box but I don't know what its algrithm really is working.