Fusion allows a BU /OU to function as a service provider for other BUs. This relatoinship is defined using the concept "Shared Service Center" .Refer to above documentation about what this is . Procurement is one of the shared services that a bu can provide another BU.
For such BUs you define a procurement business function, and BU that receives services from the procuring BU, is set us a a client BU,Requisitioning BU . and will be setup with a requisition business function.
This explicit usage of Requisitioning Business function and Procuring business function and tagging BUs ( OUs in Ebiz terminology) with one or the other function allows you to define the relationship between the BUs. This is not so explicit concept with Ebiz. Fusion from the get go started with implementing this service receiver and service provider concepts for BUs/OUs since it closely reflets ground realities SCM in the industry.