Create segments using negatives to exclude certain sets of customers (except, not, etc...) instead of just the positives (AND, OR).

Example 1, we want to send an email to customers who have purchased from a category more than a year ago. We want to be able to choose CATEGORY XYZ EXCEPT THOSE WHO HAVE PURCHASED FROM CATEGORY XYZ WITHIN ONE YEAR.
Example 2, we want to send an email to ALL CONTACTS EXCEPT THOSE WHO HAVE PURCHASED FROM CATEGORY XYZ.