The UsernameToken authentication policy adds a WS-Security UsernameToken to the SOAP header of the request on the client side. On the service side, it enforces that a user has authenticated by sending a UsernameToken in the request. The UsernameToken contains a username and password.
Talend ESB provides a template policy called wspolicy_authn_usernametoken.policy and available in the /add-ons/registry/policies folder of the Talend ESB product.
It is also applied by default to your Talend ESB Container via the following policy file: etc/org.talend.esb.job.token.policy. So if you selected the UsernameToken for your Service in the Studio, when you deploy it on your container, this policy will be picked up automatically.