INSTASPINFOCMOTORWAREGUI: Motorware code structure

Im confused about handles within the code for INSTASPIN MOTORWARE projects. Im sure its a c programming thing I am not understanding. Hope you can help me understand.

Example:

When using spinTAC components in lab13.c

 ST_Handle stHandle;

We go back to declaration within ccs5.5 and it takes us here 

typedef struct _ST_Obj_ *ST_Handle;

But when looking for _ST_Obj_ There is not struct by that name. I have tried search the whole project. I have a few questions

1. Where are its members?
2. What is this doing and how is this used?

Also stHandle is passed in here

inline void ST_setupPosConv(ST_Handle handle) {

// get object from the handle
ST_Obj *obj = (ST_Obj *)handle;

How does this work when passed into the example function above?