In Porter’s Value Chain, Service represents those activities that enhance or preserve value of the offerings over time. Examples of this are on-going support, warranty replacement, etc. Carrying forward with our shoemaker, this could be such services as resoling, a shoe shine, or even selling replacement laces.
In the context of the Business Model of Security, Service is where we respond to the changing needs of customers. Perhaps it is in tweaking reports, or in responding to incidents or finding ways to plug into to new IT management systems with existing data. It is best to think of Service as finding ways to better support your end user and to optimize the integration of your processes to theirs.