When you create a new rule, you can select either Calculation or IF-THEN rule.
Below is an example of the Calculation rule (CostPerMille = Cost / Impressions * 1000):
More complex Conditional logic can be build using IF-THEN rules. Here is an example of a rule that calculates VAT based on the Country field value (add 23% VAT to all sales in Ireland and 20% to sales in the UK).
For more details on how to create such a metric and use the IF-THEN rule, please read this tutorial.
You can also check some of our advanced rule examples in the following tutorials: