Initializing a map in a screen definition file

[Wai]

Hello,
In a screen definition file I've noticed that there are 2 styles of initializing a map. eg.

<screen name="somescreen">
    <section>
        <actions>
            <set field="someMap.aaa[+0]" value="aaa"/>
            <set field="someMap.bbb[]" value="bbb"/>

Could someone tell me what the differences are?  Are there any situations where one would be the preferred usage?

If this is documented somewhere, please give a link.

Thanks,
Wai