<WIM>
<IMAGE INDEX="1">
<NAME>FabrikamWIM</NAME>
<DESCRIPTION>FabrikamCustomWindowsImage</DESCRIPTION>
</IMAGE>
</WIM>
ok let me see if i can explain this correctly

the meta data allows you to specifiy a data image on the DVD, with a base Vista image you have 6 data images (home, premium, business, premium, ultiamte) using the metadata tags that exist for each you can specifiy which data image to apply to a installation.
doing in
imagex /info <wimfile> will give you the following information
Available Image Choices:
------------------------
<WIM>
<TOTALBYTES>1485261025</TOTALBYTES>
<IMAGE INDEX="1">
<NAME>Windows Longhorn SERVERENTERPRISE</NAME>
<DESCRIPTION>Windows Longhorn SERVERENTERPRISE</DESCRIPTION>
<FLAGS>SERVERENTERPRISE</FLAGS>
<WINDOWS>
<ARCH>0</ARCH>
<PRODUCTNAME>Microsoft« Windows« Operating System</PRODUCTNAME>
<HAL>acpiapic</HAL>
<PRODUCTTYPE>ServerNT</PRODUCTTYPE>
<PRODUCTSUITE>Enterprise</PRODUCTSUITE>
<LANGUAGES>
<LANGUAGE>en-US</LANGUAGE>
<DEFAULT>en-US</DEFAULT>
</LANGUAGES>
<VERSION>
<MAJOR>6</MAJOR>
<MINOR>0</MINOR>
<BUILD>6001</BUILD>
<SPBUILD>16406</SPBUILD>
</VERSION>
<SYSTEMROOT>WINDOWS</SYSTEMROOT>
</WINDOWS>
<DIRCOUNT>7005</DIRCOUNT>
<FILECOUNT>38647</FILECOUNT>
<TOTALBYTES>6003304564</TOTALBYTES>
<CREATIONTIME>
<HIGHPART>0x01C71B78</HIGHPART>
<LOWPART>0xD8170011</LOWPART>
</CREATIONTIME>
<LASTMODIFICATIONTIME>
<HIGHPART>0x01C71B82</HIGHPART>
<LOWPART>0xA8E3F93C</LOWPART>
</LASTMODIFICATIONTIME>
</IMAGE>
<IMAGE INDEX="2">
<NAME>Windows Longhorn SERVERENTERPRISECORE</NAME>
<DESCRIPTION>Windows Longhorn SERVERENTERPRISECORE</DESCRIPTION>
<FLAGS>SERVERENTERPRISE</FLAGS>
<WINDOWS>
<ARCH>0</ARCH>
<PRODUCTNAME>Microsoft« Windows« Operating System</PRODUCTNAME>
<HAL>acpiapic</HAL>
<PRODUCTTYPE>ServerNT</PRODUCTTYPE>
<PRODUCTSUITE>Enterprise</PRODUCTSUITE>
<LANGUAGES>
<LANGUAGE>en-US</LANGUAGE>
<DEFAULT>en-US</DEFAULT>
</LANGUAGES>
<VERSION>
<MAJOR>6</MAJOR>
<MINOR>0</MINOR>
<BUILD>6001</BUILD>
<SPBUILD>16406</SPBUILD>
</VERSION>
<SYSTEMROOT>WINDOWS</SYSTEMROOT>
</WINDOWS>
<DIRCOUNT>3205</DIRCOUNT>
<FILECOUNT>21197</FILECOUNT>
<TOTALBYTES>2553830079</TOTALBYTES>
<CREATIONTIME>
<HIGHPART>0x01C71B74</HIGHPART>
<LOWPART>0xF30A09E1</LOWPART>
</CREATIONTIME>
<LASTMODIFICATIONTIME>
<HIGHPART>0x01C71B83</HIGHPART>
<LOWPART>0x0A4715D4</LOWPART>
</LASTMODIFICATIONTIME>
</IMAGE>
so based off the following information if i wanted to write an unattended file that would install the core edition of Longhorn Server i would use
<WIM>
<IMAGE INDEX="2">
<NAME>Windows Longhorn SERVERENTERPRISECORE</NAME>
<DESCRIPTION>Windows Longhorn SERVERENTERPRISECORE</DESCRIPTION>
</IMAGE>
</WIM>
this is the same with Vista, specifying the data image is done by adding a meta data tag to the install OSinstall tag under x86_Microsoft-Windows-Setup__neutral_31bf3856ad364e35_nonSxS during the WinPE stage. you can add this metadata tag by right clicking on the install From icon under OSImage and choosing insert meta data.
on a custom WIM, you will have added the images in your own order and you will need to do an /info to find out what index it has, you will have also had to add a description tag when you captured your WIM. (you can also add it later)
hopefully i explained it well enough! let me know if there are still any questions!