Comment 7 for bug 1897369

Till, it allows quite a few things (from man capabilities):

CAP_SYS_NICE
       * Raise process nice value (nice(2), setpriority(2)) and change the
         nice value for arbitrary processes;
       * set real-time scheduling policies for calling process, and set
         scheduling policies and priorities for arbitrary processes
         (sched_setscheduler(2), sched_setparam(2), sched_setattr(2));
       * set CPU affinity for arbitrary processes (sched_setaffinity(2));
       * set I/O scheduling class and priority for arbitrary processes (io‐
         prio_set(2));
       * apply migrate_pages(2) to arbitrary processes and allow processes
         to be migrated to arbitrary nodes;
       * apply move_pages(2) to arbitrary processes;
       * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).

cups-browsed is probably just trying to renice itself, which isn't terrible for it to try, but it probably fails gracefully with this just being noise. If it does fail gracefully, you could consider an explicit deny rule to silence the log. Eg:

  deny capability sys_nice,

That said, we've normally allowed system policy (ie, those shipped in debs) to use sys_nice if they have a legitimate use case for it.