Hello,
What was in the documentation that was not clear? If you post what you found, someone may be able to clarify.
There is no definition other than the one you provide for resources that you define (i.e. special resources are local to organizations that choose to use them).
From IBM:
You can use Tivoli OPC special resources to represent any type of limited
resource, such as tape drives, communication lines, or a database. A special
resource can be used to serialize access to a dataset or to limit the number of file
transfers on a network link. The resource does not have to represent a physical
object in your configuration, although often it does