I know Ive seen these terminology on sites. I think Firestone and Slams use them. The work linear means in a straight line. Something like that would be used to descrbibe a motion that will keep the bags endplates in a straight line through its motion. I know with semi's they use a bag to keep the cab from bouncing all over the place. Its a very big arc, but Im guessing that would be close to linear. The nys inspection machines also use an air bag to lift and lower a bar to keep the car from falling between the 2 rollers on the dyno's. The motion of the bar is guided and is perfectly linear. Off axis would be a motion similar to a upper and lower control arm set-up, where the bags ends move in relation to eachother and the endplates are not kept inline through its motion. That would be off axis.
I may be wrong but I heard this term with slam specialties bags, I think that it means that the bag can be mounted in the front of rear of a vehicle. Linear might be for the rear and off axis for the front.