Question PHP-FPM when running Nginx as a reverse proxy in front of Apache

[Mike2017]

Hello,
I am running Nginx as a reverse proxy in front of Apache on my VPS (Plesk 17/Ubuntu 14.04) and I don't have PHP-FPM enabled. AFAIK, PHP-FPM is the gateway interface for Nginx for processing dynamic PHP, but with Nginx running as a proxy it never processes dynamic PHP, these are processed by Apache. Therefore, I am not enabling PHP-FPM support...
Yet, I recently came across Plesk FAQ re running Apache with Nginx (see it here) and on this page there is a picture of running services (in Nginx with Apache mode) that shows PHP-FPM enabled... Am I misunderstanding how PHP-FPM module works or I should just disregard this Plesk FAQ picture?
Sorry, I am still a newbie in VPS management/config issues, so my apologies if I'm asking something obvious/stupid

Mike