Sealed is protection against Gas, Infection, Stench, Poison, Vacc, and Tranq.
For example, Scout Eneri Dinsha 777777 is exploring a Hellworld beyond the Imperial border. He is using a StHHES-8 Standard Heavy Hostile Environ Suit -8 Sealed-22.
As he explores he encounters:
Rough Vegetation = Cut-1
Noxious volcanic gases = Stench-3 / Poison-1
Poison Thorn Bushes = Tranq-2.
None of which can overwhelm Sealed-22.