(Answer taken from the ISO citer draft, p. 11)
This International Standard supports different levels of granularity. The following recommendations are designed to encourage efficiency and promote interoperability with other naming schemes:
1) If there is an existing identifier scheme for a type of resources, for instance, ISBN, this level of granularity should be retained, which is to say that no new PIDs should be issued without very good reasons, such as for chapters. Chapters would preferably be addressed using part identifiers in conjunction with the of the book.
2) If the resource is associated with the complete content of a digital file, an individual PID should probably be assigned for this resource.
3) If the resource is autonomous and exists outside a larger context, an individual PID should probably be assigned for this resource.
4) If a resource should be citable apart from any containing resource, an individual PID should probably be assigned for this resource.
These recommendations are, however, subject to the needs of resource creators with respect to the level of granularity they deem suitable to the specific resource environment.