Composite or mixed shell boots use different plastics in different zones of the boot to get the best properties from each material. The most common setup is a polyurethane (PU) lower shell for solid power transmission to your ski, paired with a lighter Grilamid or Pebax cuff to shed weight up top. Think of it as putting the stiffness where you need it and trimming weight where you don't.
This construction really shines for performance freeride and all-mountain skiers who want resort-level responsiveness with less fatigue over a full day. You get the energy transfer of PU underfoot for carving and the lighter swing weight of touring-friendly materials above your ankle.
The trade-off is cost—these boots sit at a premium price point because manufacturing with multiple materials is more complex. If you primarily ski frontside or race, a full PU boot is simpler and proven. But if you split your time between resort laps and sidecountry or long freeride days, the composite approach can be worth the investment.