Only one relation set can be active. The current active relation set is shown in the Act R-set field of the menu panel and underlined in the Structure Browser.
1. Expand your assembly in the Structure Browser, so the relation set is visible, as shown here:
2. Click Relations and the, in the R-Sets group, click Active.
3. Double-click the relation in the Structure Browser. Alternatively, right-click the relations set and, in the shortcut menu, click Set Active.
Active.