The M/L size is designed for torso lengths between 43 and 50 cm, measured from the base of your neck (the C7 vertebra) down to the top of your hip bones. This is the most common size and fits the majority of adults, so if you're of average build, there's a good chance it'll work for you.
To check, grab a flexible tape measure and have someone measure from the bony bump at the back of your neck down to your iliac crest (the top of your hip bones). If you land in that 43–50 cm range, M/L is your size.
Getting the right torso length matters because it determines how well the pack transfers weight to your hips. A proper fit means less shoulder strain and better comfort while skiing or hiking. If the back panel is too short or long, you'll feel it—especially with a loaded pack on a long tour.