About 85% of ideas below are from speech I heard at the US Olympic Training Center 20 years ago. (I pounded this out fast at lunch so let me apologize now for typos and bad grammar)
Volume Set – a Volume Set is a ski ride where you only ski at line lengths or speeds that you expect to always make all 6 balls. A typical Volume Set starts with an opening pass or two and then 4 – 6 passes at intermediate passes. Ideally all 6 balls are rounded on every pass. Volume sets are where the fundamentals of techniques are learned and reinforced.
Intensity Set – an Intensity Set is basically a tournament set. These are the rides when you burn straight to your hardest pass. Perhaps you might back up an intermediate pass but the bulk of the skiing is at your limit. An Intensity set might comprise 6 or 12 easy balls and 12-24 balls rounded with some level of desperation. We generally ski less technically correct during an Intensity set. Often an Intensity set means a lot less balls round.
The theory that was presented at the USOTC was that an athlete could time peak performance and increase the peak potential by balancing Volume with Intensity.
If peak performance is required in a very short period of time (imagine wanting a big score at an early season tournament) a skier could do nothing but Intensity skiing. Most skiers with a bit of experience will bring their ball count up pretty fast in the spring. The unfortunate by product is that the peak potential score will be generally lower. Let me say that again => if you always take tournament rides and spend a lot of time working on your hardest pass you will get to your peak faster but your ball count will be less then what you are actually capable of.
If all you cared about was Nationals or a late season score you would want to take a very different approach. Perhaps 80% of your rides would be spent on Volume for the first 2/3s of the season. As the date of the big tournament approaches the opposite is needed. Most rides are Intensity but a few Volume rides are needed to keep things balanced.
There is not magic ratio but time spent on Volume skiing means practicing running 6 balls at a time and Intensity rides often mean practicing missing passes.
38 is the pass that I only run when I am skiing really well. About 60% of all the balls I round in a season are at 35. I ski a lot of Volume. I will not seriously shorten to 38 unless I can crush 3 or 4 35s in a row, a few rides in a row. As soon as things go to crap I go back to ride after ride working on the smoothest 35s possible.