In Custom Metrics there are two types of metrics:

Query Metrics

A query metric is a specific metric from a single data source that may or may not have a filter applied to it.

Example 1:

Clicks from Facebook Ads in Colombia

Example 2:

Clicks from Facebook Ads but for a specific campaign

Example 3:

Clicks from Facebook Ads for desktop

Calculated Metrics

You can create formulas using previously created Query Metrics or choosing metrics from Data Sources:

Example: we have an account that spends money on Facebook and Google, and we want to calculate the total spent for the account: