Setting up retention categories and applying them to archiving with Enterprise Vault can be quite a contentious issue. But, if you put that aside a question which comes up from time to time is how to set the default retention category.
Well, this is actually well thought out and well laid out with Enterprise Vault. Provisioning Groups is where this sort of thing is configured. For each provisioning group you can determine things like the policies applied to mailboxes that fall in to that group. In addition you can also specify the default retention category, as shown here:
Do you set different retention categories per provisioning group? Let me know in the comments below.