The CA had a very similar take on experience as well. It explains it like below:
“Experience measures the character's increasing skill with the abilities he already has, both inside and outside of his class.”
It also goes on to say the following:
“First of all, a character does not necessarily start out at EL 1. Nope! If the character is totally green, as most are, he has no EL at all. This is not to say he is not trained. He is presumed to have beginning skills for the use of his craft. He is just without experience.
Advancement is measured in the terms of time passed or in expeditions run. The latter is a flexible concept; the GM may well consider a quest taking weeks or months of game time to qualify as multiple expeditions. On the other hand, a milk run may not equal even one expedition. Difficulty and novelty to the characters are the key elements the GM must consider.”
The CA continued on the thinking exposed in the Arduin Adventure. It just refined it slightly, defined it a bit more. A meme of thinking pervaded that one game session equaled one adventure or expedition. The Compleat Arduin ruffled that idea; it was explicit that the GM could rule that one game night had multiple expeditions or none – it might require a stretch of 2, 3, or more to meet the requirement. Key elements expressed here are novelty and difficulty.
In the background remained the beginning chart laid out in AG I. It was refined, of course, but it mapped out that actions, especially those in the novelty, inventive, and role-playing categories were the actions worth rewarding. Gone was all mention of acquisition, at least in the sense of material items and personal power that grew from them.
If the Compleat Arduin did something different, it was that it re-introduced the idea that each class progressed at a different pace. DH did away with that in the Arduin Adventure but it was reintroduced in the CA. Warriors & Barbarians earned their EL 1 by going on 2 expeditions or 6 months elapsing. It took 4 expeditions per EL after that and then 10 expeditions for EL 5 – 8. It got much harder then, going to 15 expeditions for EL 9 & 10 and 25 expeditions for EL 11 and beyond. The other professions had it much worse: priests, wizards, illusions, druids, etc. requiring 6 expeditions just to get to EL 1 and 10 per EL afterward through EL 4 and 20 afterward to EL 12 and then 40 each for EL 13 and beyond.
The other classes were just as bad and back in was the unevenness of progression. I'm doubtful this was the idea: everything up to this point was a progression toward a simplified, singular advancement train per class. The leap backwards here was likely an introduction from the core group that was putting together CA from the notes David Hargrave left behind.