The auxiliary API is part of the CoppeliaSim API framework.
The auxiliary API is not a CoppeliaSim interface per se, but more a collection of C++ functions that can be embedded into your own code, and that operate on their own. Following auxiliary API categories are available:
Coppelia geometric routines: this is a collection of stand-alone C++ functions that offer the same collision detection, distance calculation and proximity sensor functionality as in CoppeliaSim.
Coppelia kinematics routines: this is a collection of stand-alone C++ functions that are able to handle the same kind of kinematics calculations as can be done within CoppeliaSim.
The geometric plugin for CoppeliaSim
The kinematics plugin for CoppeliaSim