This is written in the Readme, hope it helps:

Currently there is just one prefix, which is "S". Later releases may
see the introduction of more kernel variants with using other letters.

Examples:

e.g. S27x3 is a launch configuration that works well on GTX 260
      28x4 is a launch configuration that works on Geforce GTX 460
     290x2 is a launch configuration that works on Geforce GTX 660Ti

You should wait through autotune to see what kernel is found best for
your current hardware configuration.

The choice between Non-Titan and Titan CUDA kernels is automatically
made based on your device's compute capability. Titans cost around
a thousand dollars, so you probably don't have one.


Prefix  | Non-Titan          | Titan
-------------------------------------------------------
  | low shared memory  | default kernel
        | optimized kernel   | with funnel shifter
        |                    |
   S    | spinlock kernel    | spinlock kernel
        | for Kepler GPUs    | with funnel shifter