Boot sole length (BSL) is the physical length of your boot's sole measured in millimeters, and it's what your ski bindings use to clamp onto the boot. It's different from your mondo point size—the standard sizing number like 26.5—so two boots in the same mondo size from different brands can actually have different BSL measurements.
You don't choose BSL directly; it comes with whatever boot size you buy. But you absolutely need to know it if you're keeping your existing skis and bindings. When you switch to new boots with a different BSL, your bindings must be re-adjusted to fit properly. Skipping this step can lead to poor retention or even dangerous release issues.
Always check and record your BSL when getting bindings mounted or adjusted. If you're buying new boots for current bindings, bring your old boots to the shop so they can compare the sole lengths and make any needed adjustments before you hit the slopes.