Its pretty simple and self explanitory

By default it would be

invincible="false"

With an XML atribute like this, we can have truely indestructible planes

I know there is partCollisionResponse but it does not make the part immune to missiles while invincible would actually make it immune to missiles

It could be either for each separate part like massScale, scale, health, etc
Or it could be added to the plane, file and it will take effect on the whole thing like diffuseInertiaTensors

That all