Helmets with interchangeable pad systems come with multiple thicknesses of interior pads you swap out to customize fit, rather than using a mechanical dial to tighten. They're typically found on budget helmets and some youth models, and they can work fine if you find the right pad combination and don't need to make adjustments on the hill.
The downside is that pads offer less precise adjustment than a dial system, and they can compress over time, which may loosen your fit. Swapping pads also isn't something you can easily do mid-day at the lodge. If your helmet feels a bit off with the pads it came with, you're stuck until you can swap them at home.
A dial-based system is generally the better choice if your budget allows, since you can fine-tune fit quickly—even with gloves on. But if you're buying for a growing kid or just need an affordable option, a pad system can still provide a secure fit as long as you take time to get the sizing right from the start.