Decided to dive into the source code and provide a simple explanation:
Parameters:
int $which - Select which parameter to use in the CDF Binomial calculation, based on what the prior 3 parameters are.
where $which is 4:
$arg1 = p
$arg2 = sn
$arg3 = xn
returns pr
$which = 3
$arg1 = p
$arg2 = sn
$arg3 = pr
returns xn
$which = 2
$arg1 = p
$arg2 = xn
$arg3 = pr
returns sn
$which = 1
$arg1 = sn
$arg2 = xn
$arg3 = pr
returns p