An affinity mask is a bit mask indicating which CPU Core a process should be run on by the scheduler of an operating system. Setting the affinity mask for FSX/P3D declassifies its default use of CPU cores, which can lead to better application performance.
Windows 7 - FSX SP2 - The following example uses an affinity mask value of 1364 applied to a Hexacore CPU with Hyper-threading enabled utilising cores #1 to #5, whilst running FSX SP2. Note how core #0 is almost at idle, allowing it to take care of processing other system requests and processes.
Windows 10 - P3D v3.1 - The following example uses an affinity mask value of 4095 applied to a Hexacore CPU with Hyper-threading enabled running P3D v3.1. Note how logical core #0 thread #0 is at 100% dedicated to P3D rendering tasks. The other logical cores and threads load balance the other P3D processing requests.
Always backup your data before making changes to configuration files. This software has no guaranties or warranties and is used at your own risk!