Separation of Concerns: the idea that a software system must be decomposed into parts that overlap in functionality as little as possible.

The application of SOLID/Single Responsibility Pattern such that Services focus on their primary area of expertise/concern and delegate to other Services for specialized assistance, rather than use private helper methods. E.g: IActiveDirectoryService may specialize in using LDAP to communicate with Active Directory, and will use tracing, caching, and retrieve AppHost settings, but instead of encoding this logic within itself, will use methods available in ITracingService, ICachingService, and IAppHostSettingsService. See SRP.

It's pretty much the same concept as SRP