Most tech bindings use a heel gap that allows a ski to flex as the heel pins sink deeper into the rear boot fitting. These spacer tools will help you set that gap accurately when you're adjusting your binding for different boots. Made by Dynafit but 4mm is a common gap size for other bindings as well.
4mm Heel Spacer - The yellow plastic stick used to set the gap on the Low Tech Race (1.0 & 2.0) and some older Dynafit touring bindings.
5.5mm Heel Spacer - The white plastic feeler gauge used to set the heel gap on Radical 1.0, Vertical, Speed Turn, & Superlite bindings.