Controlling who can request Leave or Sickness absences is defined in the Team profile. Settings for each are slightly different.
For Leave, the options are to allow only the user themselves to submit a leave request, or to allow managers to submit leave requests on behalf of their team members.
For sickness absences, these same options are available, but in addition options are provided to allow anyone in the team, or anyone across the organization to report a staff member as away from work. This is to allow for those occasions where sickness absences are reported in a more ad hoc/pragmatic fashion that those with formal procedures to follow.
Note that, in addition to the above there is a Backfill option for creating absences without them following the workflow approval process.