A longstanding debate in IT service management is the relationship between Service and Application. Readers of my book and this blog know that I see an Application as a subtype of Service, not as a component of.
The alternative view is well articulated in various ITSM and ITIL materials: Applications are technical and too low level for the customer to care about. A hierarchy is proposed:
However, if one examines the reality of Application Portfolio Management: it is targeting a much higher level concept. APM requires information on financials, operational metrics, customer impressions, and so forth. See especially Bob Benson's book and product literature for Prosight and similar products. Also, a chapter from the excellent Handler/Maizlish book on IT portfolio management is available for free here. Note that Handler and Maizlish do not even use the term IT Service.
We would not be managing applications as a portfolio if they were mere subcomponents of something bigger (i.e. Service). We'd be managing the "something bigger."
The capture of customer perception in particular points up the contradiction. In the classic ITIL sense, the Application should be "invisible to the Customer." So why do application portfolio management products focus on customer satisfaction? Because application is a type of IT service - not a purely technical back-end function.
Now, this does not preclude the existence of process-based IT services, such as Customer Credit Authorization, that are essentially transactional and may cross multiple systems.