The JavaTest Agent can run in active or passive mode:

Passive | JavaTest initiates connections to a passive agent as needed. Passive agents are slightly simpler, but less flexible because they require you to specify the test machine as part of the test configuration, not at the time you run the tests. Passive agents do not allow you to run tests in parallel.
Active | Active agents initiate connections to JavaTest as available. Active agents allow you to run tests in parallel using many agents at once. Active agents allow you to specify the test machines at the time you run the tests.

Unless your decision is dictated by security restrictions on your platform, active agents are recommended.

For more information about JavaTest Agent, refer to the JavaTest User's Guide or to the JavaTest online help.