The meaning of organizational unit is context dependent. For example, it can be an individual company or an individual non-governmental organization. If you think about a corporation, each business unit may be seen as an organizational unit. If you think about a hospital, each main service, blood services, infectious-contagious, may be seen as an organizational unit.
For each organizational unit there are specific functions, normally identified in an organizational chart. Like warehouse manager, like seller, like quality controller, like line operator.