See the Javadocs in <EPM_ORACLE_HOME>/common/docs/ReportingAnalysis/SDK/javadoc.
Each artifact stored in EPM Workspace contains a unique identifier that represents the artifact. Whereas multiple artifacts can be stored in a folder with the same name and metatype, they are distinguished from each other by the universally unique identifier (UUID) assigned to the artifact. This interface represents the UUID when invoking Interactive Reporting SDK methods that require an artifact ID.