Not sure if it’s the widely used definition, but I think of affectable as anything in our future light cone, with accessible as just the bits which we could get something (e.g. a von Neumann probe) physically to before the expansion of the universe takes it away from our reach, which is a smaller bubble because we can’t send matter at light speed, our probe would have to slow down at the other end, and the expansion of the universe robs our probe of velocity relative to distant galaxies over very long periods.
Not sure if it’s the widely used definition, but I think of affectable as anything in our future light cone, with accessible as just the bits which we could get something (e.g. a von Neumann probe) physically to before the expansion of the universe takes it away from our reach, which is a smaller bubble because we can’t send matter at light speed, our probe would have to slow down at the other end, and the expansion of the universe robs our probe of velocity relative to distant galaxies over very long periods.
Edit: The FHI has a relevant paper: https://www.fhi.ox.ac.uk/wp-content/uploads/space-races-settling.pdf