A Fair Use Policy (FUP) is the small print on most "unlimited" or "all you can eat" mobile plans. It caps your usage — particularly while roaming — to prevent customers using their home plan as a long-term substitute for a local SIM.
Typical EU FUP
Most UK carriers cap data roaming in the EU at somewhere between 5GB and 25GB per month, even on otherwise-unlimited plans. Once you hit the cap you either get throttled to slow speeds or charged per MB.
Why it exists
Networks pay each other for roaming traffic. If a customer roamed permanently on a £20/month plan, the home network would lose money. FUPs are the limit that keeps the maths working.
How to check yours
Carrier websites publish their FUPs — usually buried in roaming pages. Fuse publishes its FUP openly on every roaming-included plan page.