ITIL talks about three types of Service providers:
a) Internal Service provider – This represents IT departments/service providers embedded within business units in organizations.
b) Shared service unit – which provides services to multiple business units within a corporate and
c) External service provider – which represents IT departments/Service providers external to the business organization.

Now, there is no ambiguity regarding these definitions.  But I used to get a bit doubtful when somebody asks a clarification on the same such as:

Look at a scenario of a service provider who provide services to internal as well as external customers. How do we fit them into this classification:
a) As a shared service unit? – I used to agree with this initially; but was not really convinced fully.
b) As a combination of internal and external? or shared and external? – May be. But that also didnt look convincing to me.

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