This is from the sp3 client help.
To change an item's Retention Category
Display the item�s properties
Click the Enterprise Vault tab. The Enterprise Vault archiving settings for the selected item are displayed.
Click the Retention Category arrow and select the one that you want to use. The Description and the Retention period display additional information about the Retention Category that you have selected.
Tips
It is recommended that you keep items with the same Retention Category in the same folder. This enables you to assign the Retention Category to the folder, so it applies to all the items in the folder.
You can assign a Retention Category to an item when you store an item in a Vault yourself.
Note
If the Retention Category setting is unavailable it has been locked by your Administrator.