A population is a group of Virtual User types. Several different populations can be configured to test the application using different business and network behaviors. For example, a web store site can be tested by simulating 70% of users browsing the catalog (visitors), 20% carrying out a purchase through to its end (buyers) and 10% being administrators. Besides, network constraints can be configured to test the application under realistically simulated conditions.
The Design section includes the Populations tab where you can create and configure the populations for the test.