RAM and CPU are not available in Resource Controller (cgroups) on Debian 8 but on Plesk Administrator Guide there are no limitations indicated for Debian 8.
Cgroups has limited functionality on Debian 8 OS. Debian 8 kernel does not support the cgroups CPU and RAM resource usage limitation. This is limited on a kernel side. Here is the detailed information about the issue on Debian side: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=789019
Plesk documentation does not reflect these limitations so far, so it is confirmed as bug with ID #PTD-1295 which will be fixed soon by updating Plesk documentation.
As a workaround, install the newer kernel version from the debian-backports repository using the steps described in the article below: