Funnel accounts come with three default metrics: Cost, Clicks and Impressions with related rates like CPC. Each of these uses a specific set of built-in rules for grouping your data which you can override to suite your needs.
The metric used varies depending on your connected sources. You can see what rules are used by viewing the metric details.
You can also create new metrics based on these rules by using a template, for instance if you want different types of cost.