how can virtualsystem match File

Hi, everyone!

As we know, there are many File in the Reference section. In addtion, an envelope may contain several virtualsystems, each of them describes a virtual machine.

But, how can we know which image file belong to which virtualsystem?

for example

<File ovf:id="base" ovf:href="base.vmdk" ovf:size="180114671"/>
<File ovf:id="webdelta" ovf:href="webdelta.vmdk" ovf:size="123413"/>
<File ovf:id="dbdelta" ovf:href="dbdelta.vmdk" ovf:size="343243"/>

<VirtualSystem ovf:id="WebTier">

how do you know which vmdk does the virtualsystem describes?

Thanks for your answer!

Peter