How to best model a shared service?

Hi,

Our enterprise has an overarching corporate entity, with dozens of small business units.  To the side of all these business units we have our "shared services".  For example, HR.  

I need to model the business processes, etc., of our business units, and I need to be able to model two things:

1. What a shared service looks like (org, business services provided, business processes, capabilities)
2. How it looks when a shared service has a function they deliver on one of the business unit processes

I can obviously model the roles and organisation of the shared service easily enough.  I can also model its service landscape.  I'm wondering how to then present that as a business service or something in the process model, if I should be using Business Service at all, or even how to nicely distinguish internally provided products and services to external ones to customers.

Does anyone have any tips?

Thanks,

Chris