ISO/OSI Reference Model: Entities and Modular Open Systems

[Harald]

In case this is the wrong usenet group to ask my question I would be
grateful if you could tell me where I should better ask my question in
order to get a helpful answer.

I'm not quite clear on some ISO/OSI terminology as used in protocol
definitions and which stem from the OSI Reference Model (X.200). In
particular, while a subsystem consists of possibly multiple entities
I'm not clear what happens in case a subsystem is located within a
*distributed* (or modular) open system. Can a single entity spread
across multiple modules of such an open system? Or do multiple
entities of the same type cooperate together in order to make them
appear as a single entity spread across multiple modules of an open
system?

-- Harald