How can I set exceptions for particular usages of product groups?

In the Manage Product Group Usage page, select a usage.

For example, select the Base usage. Click the product group in the Product Groups tab, select the usage in the Modes tab, and use the other tabs to specify the exceptions for this usage.

You can use this method to modify the appearance of a product group in minor ways for different uses. For example, you might want a product group to look slightly different when it's used for lead management and for campaign management.