PATHCPU means native c++ driven rendering, no opencl involved.
PATHOCL lets you use all opencl capable devices you have, this can be gpu and cpu.
The benefit of latter is you can use cpu and gpu at the same time for boosting rendering.
If opencl turns out to be a beast, its often recommended to first test only with ocl cpu device, cause this
is almost the stablest testcase, whereas gpu devices failure can be also from bad drivers.
( That said also cpu has a vendor ocl driver, but we very rarly saw any problems from them )
Another hint of working cpu but failing gpu can be a too huge memory print from a rendered scene.
Jens