HFOs such as R1234yf and R1234ze are chemically similar to HFCs but break down in days rather than years once released, so their GWP is very low. Most pure HFOs are classed A2L: lower flammability, much harder to ignite than hydrocarbons but not completely non-flammable.
In commercial refrigeration, HFOs often appear in blends mixed with HFCs. Blends such as R448A and R449A were developed as lower-GWP replacements for R404A in freezers and cold rooms; their GWP is still well over a thousand, roughly a third of R404A's, but they keep non-flammable classifications. Some blends have temperature glide, meaning they boil over a small range, which technicians must account for when reading superheat.
HFOs and HFO blends are typically more expensive per kilogram than hydrocarbons, and some have specific handling requirements. They are a useful option where a non-flammable or lower-flammability gas is needed, such as larger remote systems.