TL;DR;
Nike used to put SF6, a greenhouse gas which causes 24300 times more warming than CO2 over 100 years, in the “air” cushions in their shoes. Nike stopped, and sold carbon offsets based on the counterfactual of what the world would be like if they kept putting it in shoes. These offsets are part of the “buffer pool” to compensate for the fact that some offsets turn out to be bogus or simply fail due to unanticipated consequences.
I think that nitrogen is what they’re using now. It’s readily available, cheap, and fairly easily contained.