Storing a single value is like

<param>value</param>

Storing a multiple is like

<param>
    <value>value 1</value>
    <value>value 2</value>
</param>

But the following value

<param/>

could be a empty single value or an empty multiple one

As the storage must be able to know if multiple or not, we have to store an attribute on it

<param multiple="true"/>