Transnational (or global, international, etc.) history is simply any sort of history that analyses relations between a region and really any other area. So yes, it applies to many different objects of study on numerous different levels: whether in fleshing out the history of an idea in comparative approach, in describing how these were modified in diffusion, in identifying perhaps physical trade or information networks and other modes of transmissions, in exploring notions of cosmopolitanism or what 'global' meant to contemporaries, and so forth. If one were to analyse Christianity through a transnational lens, for instance, it might mean thinking about its geographical spread, or trying to develop an understanding about how different communities might have received certain theological interpretations, etc.