 There is a debate in the literature concerning how the principle of relativity, and more generally the global space-time symmetries, should be understood. On one approach, the significance of space-time symmetries is captured by considering the structure of a theory through transformations on its models, those models consisting of differentiable manifolds endowed with various geometric objects and relations (see Anderson, 1967, and Norton, 1989). According to Brown and Sypel (1995) and Budden (1997), this approach fails to recognise the central importance of effectively isolated subsystems, the empirical significance of symmetries resting on the possibility of transforming such a subsystem (rather than applying the transformation to the entire universe). For further developments in this debate, including applications to local symmetries and to gauge theories, see Kosso (2000), Brading and Brown (2004), Healey (2007), Healey (2009), Greaves and Wallace (2014), Friederich (2015), Rovelli (2014) and Teh (2015, 2016).