It's a blend of refrigerants with a little butane. M099 is a trade name, R438A is the actual refrigerant if you want to search for a lot of information.
Bottom line is it's a replacement for R22. Note that is replacement,not for topping off R22 in a system . The price is cheaper but there's a greater capacity loss than R22 as it gets hotter out and power consumption is higher at all temps where the AC would be on. Trane recommends replacing 20% of the mineral oil in the compressor with POE oil to aid in oil return to the compressor.
IMO we're better off sticking with R22 right now and paying the extra $6 a pound (wholesale). R22B (known as NU22) and 421A are also retrofit refrigerants used in place of R22. The R421A is the closest match to R22 but 22 is still a better value IMO.