Use services to code business processes. Microsoft Dynamics AX 2012 supports the following services:
Document services are query-based services that can be used to exchange data with external systems by sending and receiving XML documents. These documents represent business entities, such as customers, vendors, or sales orders.
Custom services can be used by developers to expose any X++ logic, such as X++ classes and their members, through a service interface.
System services are provided by Microsoft Dynamics AX. System services include the Query service, the Metadata service, and the User Session service. System services are not customizable, and they are not mapped to any query or X++ code.
Developers should use the existing services exposed in the Microsoft Dynamics AX base layer. Expose any new business processes through services.