SCNShadable behavior for SCNGeometry and SCNMaterial

Both of SCNGeometry and SCNMaterial does conform SCNShadable protocol(https://developer.apple.com/documentation/scenekit/scnshadable), but I can't figure out what is logical difference between setting program property to geometry and set to material. What if I'll set different programs to SCNGeometry and for its material(single for simplicity)?