UPF 50+ blocks at least 98% of UV radiation, making it the highest level of sun protection available in fabric. It's especially important for skiing because snow reflects up to 80% of UV rays back at you — so you're getting hit from above and below simultaneously.
You'll want UPF 50+ if you're skiing at high altitude where UV is naturally stronger, heading out for spring skiing when the sun is intense, or if you have fair or sun-sensitive skin. It's also a smart choice for full days on the mountain where exposure really adds up.
The good news is there's no downside to choosing UPF 50+ — it doesn't affect breathability or comfort. Many quality balaclavas now include this rating as standard. If you mostly ski on overcast days or at night, you could get by with less, but for most resort and backcountry skiing, UPF 50+ is worth having.