As explained in Documents, Part Studios, Assemblies, and non-native files imported into Onshape documents are represented in tabs in the user interface. Click a tab to select it and make it active. Only one tab is active in one browser tab at a time, but you can open any tab in a new browser tab to have multiple active tabs open. In addition to working in a tab, you can perform actions on tabs. Right-click on a tab to access the context menu for that tab.

Right Click Menu for document tabs