How does Aras handle the action(s) on a collection of items in an AML?

I have written a server method that uses some custom XML-like AML to implement an inventory reconciliation.  I would like to pass in a collection of items for this method to act on.

When I implement a server call like:

<AML>
    <Item type="Part" id="1" action="mymethod"/>
    <Item type="Part" id="2" ./>
</AML>

is "this" the collection?  Or is the correct approach to pass the collection within an item?

<AML>
    <Item type="Part" id="x" action="mymethod">
        <Item type="Part" id="1" ./>
        <Item type="Part" id="2" ./>
    </Item>
</AML>

Or, is it generally considered best practice to have the server method called independently for each item?   

Thank you!