An adjustable back length system uses a slider or Velcro mechanism that lets you change where the shoulder straps and hip belt sit on your body, effectively resizing the pack's fit. This is especially useful if you fall between standard sizes—say, your torso measures right on the border between S/M and M/L—because you can dial in the exact fit rather than compromising.
It's also a smart choice if you share a pack with a partner who has a different torso length, or if you like the flexibility to fine-tune your fit with different layering setups. The trade-off is that adjustable systems add a small amount of weight and a bit more complexity to the pack's construction.
For most skiers and riders, a fixed size that matches your measured torso length works perfectly well. But if you're between sizes, adjustability is worth it—a properly fitting pack transfers weight to your hips instead of straining your shoulders, which makes a real difference on long touring days.