Rallycar national registration rules

[Sulland]

I see that in some countries you do not need a numberplate at all to compete.
In other countries you can use whatever nations plate and still compete, and have valid insurance.
Some places you need to have a valid national registration and plate to be able to drive the car for more than 6 months, and keep insurance on the car.

Easy to see from the eWrc database, that most cars in the eWrc database keep the plates from the country where the car was produced, still after the car is sold on to a new owner.

Are there any common rules in EU, or other unions, or is this only up to the nations?
Is it ok in many countries to insure a car on foreign plates, or is it special rules for rally cars?