These airlines have an array of fare/seat classes, and each has its own nomenclature for the various levels.
This is US domestic flight classes.
Classes listed in ascending order
Alaska- Saver - Main - Premium - First
AA- Basic Economy - Main - Main Plus - Main Select - First
Delta- Main Basic - Main Classic - Main Extra /// Comfort Classic - Comfort Extra /// First Classic - First Extra
UAL - Basic Economy - Economy - Economy(Refundable) - Economy Plus - Biz/First
Southwest- Basic - Choice - Choice Preferred - Choice Extra
Are the airlines trying to differentiate themselves from the others or working to create confusion so they can take advantage of the public. I have no insight on their intentions and have no belief on the subject.
Just an illustration of how precision ( or lack thereof) in the wording can have significant consequences.
Happy Holidays to All