The most common boot sole norm for inbounds ski boots is ISO 5355. This sole is often called the “DIN” sole, or the “alpine” sole. It’s designed with hard plastic under the toe and heel to allow for smooth, consistent releases from alpine bindings. It doesn’t walk as well as norms with rubber soles, or rocker, but ISO 5355 soles will work in a huge variety of bindings including traditional alpine bindings, WTR bindings, and GripWalk Bindings. It’s important to note that the standards for ISO 5355 stipulate that the boot cannot have tech fittings. So while some boots on the market have the same shape sole as an ISO 5355 boot, if they have tech fittings they’re not strictly indemnified to work in pure ISO 5355 bindings (but will work in Gripwalk and MNC bindings).