A service in this context consists of two things. The service offering and the service request. The service Offering describes what is included in the service. What parts of the service is optional vs mandatory and the cost associated with the consumption of the service or any parts of it. All available services should be found in the IT Service Catalog. This service is often what is used for follow-up with the customer.
The service request is how you trigger a Move, add, change or delete (MACD) request and there are usually multiple different requests depending on what you want to do and they all are connected to the same service. The service request can be requested by the customer on behalf of a consumer or the consumer can initiate it to be approved by the customer if applicable. the consumer is the one that will use the service or parts of it. The service requests should be found in the IT Service Request Catalog.
A customer understands the warranty (when available) and usability (what functionality) of a service that could be available for a consumer by reading the service offering in the Service Catalog. the consumer initiates a service request which states what and how the service is to be consumed, and if required, the customer approves or denies this service request in the Request Catalog.
In the service catalog you should find the service offering and the service requests you should find in the Request Catalog. Optimal is if there is a relation inbetween them to clearly show what requests are related to what services. The service offering is the description of the service and the service requests are the way you can interact with the service by MACD consumers, capability, functionality etc.