The types of services play and important part of you "services hierarchy". Understanding what is a core service, enabling service and enhancing service requires understanding your customer and value that they expect to receive from IT.
A core service is a service that MUST be there for any value to be delivered. A cell phone for example, must provide the ability to communicate. Be it through a voice call or a text message, cell phones must be able to communicate. A burger joint must serve hamburgers. If either one of these are not true, neither the cell phone nor the burger joint provide value.
Enabling services are those that MUST be in place for the core service to be delivered. They are often not "sold" to customer and the customer does not know that they are there or takes them for granted. For a cell phone, some enabling services may include the server services, cell tower services and directory services. For the burger joint, enabling services include electricity and air conditioning on a hot day.
Enhancing services are those services that are the "wow factor". Have you every been shopping for a product and realized there is a feature that you didn't know about? Its was probably cool, right? Enhancing services are services that enhance the value of a service and are not absolutely required. You must be careful when providing enhancing services because it increases the cost of the service and may not result in increasing the value as much.
For a cell phone, the ability to download an app is an enhancing service. At the burger joint, having 50 sauces to choose from on your burger may be an enhancement.
Types of services change over time. Some core services may start out as enhancing services then be required by customers and their needs change. Text messaging on cell phones was once an enhancing service, now it has become a core service. Even my mother texts me now more than she calls me. The same is becoming true for downloading apps. It will soon become prevalent in cell phones.
Another thing to keep in mind is the second sentence above. It requires understanding your customers and their needs. You probably objected when you read that downloading an app is an enhancing service. It may be a core service to you. Different customers have different needs. Even different demographics of customers have different needs. Some cell phone users don't require the ability to download an app. However, if the customer is an app developer, it will definately be a core service.
This is the way our society moves. Things that are new and innovative today will be a requirement tomorrow.