How much a part grows or shrinks with temperature (ΔL = α·L·ΔT) — and, if it's fully constrained, the stress that builds up. Pick a material or enter your own coefficient.
ΔL = α · L · ΔT. If the part can't move at all, that strain turns into stress: σ = E · α · ΔT — which is often large, so thermal growth must be designed for (expansion gaps, slots, flexible mounts).