Question special

The BMJ analysis points to something that wasn't obvious to me: the difference between "target" and "collateral" selection for resistance.
I wonder whether it's worth explaining this distinction further for nonspecialists. As I understood it, the "collaterals" are commensals that can become bothersome after acquiring resistance or R factors from what the authors call "professional" pathogens. Can these R factors migrate across different families of bacteria?