Tags are used to categorize data sources. They can be thought of as a categorization instead of an identifier for a particular Data Source. This allows the System Administrator to build a taxonomy describing the data being ingested in the system. A Data Source can have multiple tags assigned to it. The example below shows a data source with a multi tag:
This data has been categorised as transaction data, the www tag describes the data as coming from the application server and the uat tag describe the environment the data is originating which happens to be the test environment in this example.