A thermo-formable stock liner is a heat-moldable liner that comes included with most mid-range ski boots. Unlike premium aftermarket liners (like Intuition), these are designed to be heated and shaped to your foot once or twice at a ski shop. The process takes about 15–20 minutes and gives you a noticeably better fit right out of the gate compared to non-moldable liners.
For most all-mountain skiers, a thermo-formable liner hits a sweet spot between comfort and cost. It provides solid heel hold and moderate customization without the premium price tag of custom foam or aftermarket liners. Just keep in mind that these liners tend to pack out (compress and loosen) a bit more over time than higher-end options.
The key is actually getting them molded—don't skip the shop visit! And if they start feeling loose after a season or two, you can often get them re-molded once, or consider upgrading to an aftermarket liner later rather than replacing the whole boot.