Everywhere I’ve seen the same confusing “explanation” and they make it even worse.

The correct understanding is I suppose:

 
high priority = lower priority value

Another confusing thing, see the output of:

# renice -n -12 $PID

is: old priority 0 new priority – 12?

When in reality is the nice value!!! because the old priority is 20 and new 8!

If your system is started via systemd (most distros, including many embedded systems such as NAS, are slowly moving towards systemd as the de facto standard way of booting a Linux server), you will very likely manage services by configuring files inside /etc/systemd/system or thereabouts. You can set the nice level at boot time by adding the directive Nice=XX where XX can be set from -20 (highest priority) and 19 (lowest priority) — the default value is zero.

Source: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Nice=

Sorry, I meant permanently. As I understand it Renice and Nice only are in effect until the next user session. I want the processes to always run under that priority without me issuing a command every time.

@Will

Just run this command specifying your process:

$ sudo nice -5 process_name arguments1 arguement2 ....

If it’s already running, renice it like this(replace 1055 with the actual process ID):

$ sudo renice -n  -12  -p 1055