In the latter case the background magnetic field is truly external to the system under study. In field theory applied say to condensed matter physics or atomic physics, where one is studying not the theory of the entire universe but some system within it, the concept of an external background makes sense. However when one studies theories such as the standard model coupled to gravity, which purports to be an effective theory of the entire universe, strictly speaking there is no meaning to the concept of an external background.

Of course the standard model is usually formulated in a particular metric background - namely the flat one. Here the reasoning is that for small standard model field energy densities, the Einstein equations are solved by the Minkowski metric. In principle it can be studied in a different metric background for example a cosmological background (FRW, deSitter etc.). However this smooth gravitational background field assumption is certainly expected to break down close to the Planck scale ($M_{P}\equiv1/\sqrt{8\pi G_{Newton}}$). At such high energies one expects a significant contribution from virtual quantum gravity processes (such as the creation and annihilation of blackholes, wormholes etc.) and the entire framework will break down.

String theory 