An affinity mask is a bit mask indicating what processor(s) a thread or process should be run on by the scheduler of an operating system.
[1] Setting the affinity mask for certain processes running under Windows can be useful as there are several system processes (especially on domain controllers) that are restricted to the first CPU / Core.
So, excluding the first CPU might lead to better application performance.
Thread affinity in Microsoft Windows can be specified with the SetThreadAffinityMask function.
[1] Forcing of each OpenMP thread to distinctive cores in Windows can be accomplished by means of the following C code: