Sims2Pack
18 - Header (Sims2 Packager 1.0) 4 - Data Offset
Then comes the XML data...
<?xml version="1.0" encoding="UTF-8"?> <Sims2Package type="Object"> <GameVersion>2141707388.153.1</GameVersion>
<PackagedFile> <Name><![CDATA[XXXfilenameXXX]]></Name> <Crc>552674faeebf57f3a8f0550f690ac902</Crc> <Length>8903470</Length> <Type>Object</Type> <Offset>0</Offset> <Description><![CDATA[XXXdescriptionXXX]]></Description> </PackagedFile>
</Sims2Package>
Following this comes the file data according to the offset and size info in the XML header. Also, the offset is relative to the end of the XML data.
Obviously the "start/stop repeatable" tags don't appear in the file, thats just for your benefit. This archive format is used for object downloads from www.TheSims2.com, and as such contains *.package files.
Here's a Sims2packDtd Sims2Pack DTD.