I think you misread.
It is 1/3 for apprentice/full cave.
Basic/intro Cave is 1/6.
Standard "cave" tanks should be at the intro level - around - LP85 or HP100. This is a general rule of thumb not a bright line. It's a function of both weight and volume for penetration and training.
After some time & comfort - once buoyancy is nailed down you can migrate up to LP95 or HP120s.. later after full cave use of stage bottles & deco cylinders will also contribute to penetration options.
If you are used to 108's and are bringing them, then by all means. But if you are renting, MOST cave shops will have certain tank sizes 85s, 95s and 120s are very common cave cylinders. Being consistent in cave diving is important so training in one tank weight is helpful and reduces the need to recalibrate ever tank swap.
Dan-O