Defining an information system is important for this process. If we look at an Enterprise Resource Planning tool, it is likely distributed across several servers and pulls information from many other applications. The National Institute of Standards and Technology (NIST) in its publication SP 800-60 suggests that a system is generally bounded by a security perimeter.
Another way to think about the boundary of an information system is to be a bit more result oriented and describe the system by what you are seeking to address through a common set of controls and policies. In the ERP example, the application itself and the servers running the ERP application may be the system, but the other applications, from which it draws some information, may have their own owners and controls and are not actually defined as part of the ERP system. On the other hand, you would not describe each server used to run the ERP application as a separate system since they all presumably would have the same perimeter boundary and managed through the same control set.