M can and does get used for heating systems all the time, but as mentioned it is thinner wall. I use type L on everything including domestic water. M isn't even an option for most mechanical inspectors, if the system gets inspected. Needs to be L or K, which it thicker than L by about the same margin between M and L. ACR is the same thickness as L, but clean and registered for refrigeration usage. Much higher priced but will serve anything that L or M is suited for. As far as compressed air lines or anything high pressure or subject to wicked vibrations, lines "should" be brazed using sil-phos. Just like your central A/C lines; as the A/C unit is quite similar to an air compressor. Regular soft solder works just fine for compressed air lines if you know what you're doing