The sealing process is done with a few different materials.
One is polysulphide, which is a 2 part liquid kind of rubber mix. They get mixed in the machine then applied to the perimeter of the two pieces of glass, against the spacer bar, to provide an airtight seal between the two pieces.
Another is silicone seal, same process, different material. Silicone seal is a better choice for things like horizontal glazing, as in skylight units in houses.
Silicone sealing is better than polysulphide for durability and longevity.