What is the difference between Jaxb nillable and required, is it not very clear in meaning?

Question

Java

Let's say there is an element in the circuit that may or may not be.
And I accept it

<user>
<car>1</car>
</user>
@XmlElement

Suppose not everyone has a car, or they simply forgot to send it. More like required = false, but there is also nillable, which confuses me