A multi-network SIM (or multi-network eSIM) is a single SIM that connects to more than one mobile network. Where a traditional SIM ties you to one carrier (and therefore one network's coverage), a multi-network SIM moves between networks automatically.
How it works
Your eSIM holds profiles for several networks (in Fuse's case: EE, Three, Vodafone and O2). Your phone monitors signal quality from each and switches to whichever is strongest in your current location. The switch is invisible to you — same phone number, same data session.
Why it matters
UK "99% coverage" claims by single networks are misleading: 99% of the population may be covered, but only by somewhere. In any given town or postcode, one network is usually weaker than another. Multi-network SIMs eliminate that gap by always picking the best of four.