This functionality is available on Onshape's browser, iOS, and Android platforms.

An Onshape Assembly tab is where you define a hierarchical structure of part and subassembly instances of an Assembly. It is also where you define degrees of freedom and relations. You are able to have more than one Assembly tab in a document. One Assembly can instance another Assembly as a subassembly, and/or instance a part directly. You are able to instance parts from the same document or other documents to which you have permissions (and that are versioned).