Likelihood based active set selection method

Which method is used for 'ActiveSetMethod','likelihood' input for fitrgp? http://de.mathworks.com/help/stats/fitrgp.html#input_argument_namevalue_activesetmethod

Is there any source material that describes the process? Or a brief explanation here would be enough as well.

[GO]

After some search I have found the related source code. It should resolve the problem. For future reference, it is located in: "...\R2015b\toolbox\stats\classreg\+classreg\+learning\+gputils\selectActiveSet"