Core Services is basically data services. Typically they evolve to become the master repository for the most important business objects in the enterprise. Typical examples are Customer and Product. You will also fint that it often makes sense to split a service for a domain object into a CRUD (master) service and a accompanying Query Service to keep the complexity to a manageable level, and to allow a more feature rich query interface.

A Master-Service is established and made available for all systems in our enterprise

• The Master-Service has the responsibility to collect and aggregate master-domain-objects across the under laying systems
• The Master-Service will master merging of elements in e.g. customer objects on the field level, based on a rule set of which changes are allowed in the different systems (a contact person with invoicing responsibility is 'owned' by the order system, and will not be updated by a mistake by the portal or the CRM system).
• The Master-Service can be expanded with its own persistence/storing to make sure of optimal uptime, so SLA will not be impaired if one or several of the systems falls out.
• The Master-Service will establish triggers in the different systems, and will there for detect by them self out-of-bounds updates and merge this by applying the same rule set as described above.

See EDR and EDR-MDS for more in-depth of the mastering strategies.