If you use Analytics as the reporting framework for an activity (A4T integration) , all reporting and segmentation for that activity is based on Analytics. Using data from Adobe Analytics in Target (A4T) makes it possible to create tests based on Analytics conversion metrics and audience segments. It also enables you to use Analytics reports to examine your data and results.
Please see the following link for more information :
Also, you can use predefined New Visitor , Returning Visitor Audience .
A visitor is included in the "Visitor: New" segment if it is the visitor's first time visiting the site, the first time since clearing cookies, the first time since the mbox.js file was installed, or if it has been two weeks since the last visit. (For information about extending the two-week profile lifetime, talk to your Adobe consultant.)
The visitor is included in the "Visitor: Returning" segment if the user previously visited the site, left for at least 30 minutes, and returned to the site again with the same cookies. As long as a visitor returns within 14 days, they will be a returning visitor.