Select the Case Key (Data Mining Wizard)
Use the Select the Case Key page to select the attribute of the source cube dimension that will be the key of the mining structure. This attribute must be in a hierarchy or have its attribute hierarchy enabled.
This page will appear only if you have selected From existing cube on the Select the Definition Method page of the wizard.
For More Information: Data Mining Wizard (Analysis Services - Data Mining), Create a Relational Mining Structure
Click the attribute that will be the key of the mining structure.