It's really a useless metric if you use it to compare one airline vs another. For instance, some airlines outsource some of their maintenance eg. heavy checks, engine overhauls etc. while other airlines will do that work in-house. Then some airlines outsource their ramp or passenger handling, while others do it all themselves. It's only useful if you compare the same airline doing the same things period A vs period B. There are no really good metrics to compare airlines ... even RASM and CASM are affected by Average Stage Length (think manpower required to fly long haul international vs short haul domestic) so the only metrics that really count are RASM vs CASM in the same airline. I will guarantee you Porter's CASM is sky high vs AC or WJ, but their RASM will also probably blow AC or WJ away. Airlines with First Class, Executive Class, Lounges, Frequent Flyer programs etc. will all probably have higher CASMs vs those who don't, but then, hopefully, their RASM will be much higher as well.