In general scriptlets run with the same priority as rpm itself. However on legacy SysV init systems, properties of the parent process can be inherited by the actual daemons on restart. As a result daemons may end up with unwanted nice or ionice values. This plugin resets the scriptlet process priorities after forking, and can be used to counter that effect. Should not be used with systemd because it's not needed there, and the effect is counter-productive.
There are currently no options for this plugin in particular. See rpm-plugins(8) on how to control plugins in general.