User Segments in both ATG and Endeca.
In ATG, user segments are created and managed from BCC (Targeting and Segmentation > User Segments). User segments in ATG are used for personalization for specific group of users.
User Segments in Endeca can be used as triggers for triggering the appropriate content configured in Experience Manager. In Endeca, User Segments can be created from the Endeca Workbench.
User segments in ATG and Endeca work in tandem to deliver a personalized experience to users. This is how it is achieved:-
Whenever ATG invokes the Endeca Assembler, ATG passes all the User Segments that are applicable for the current Profile. If you want these ATG user segments to act as a trigger for any content in Endeca Experience Manager, you need to define User Segments with the same name (as in ATG) in Endeca also, and use these User Segments as triggers for your target pages.
A good example use case would be :-
“You want a different home page for logged in users and anonymous users on your store”
1) Create a User Segment in BCC for logged-in users, and publish it.

2) Create corresponding user segment in Endeca
3) Now make sure that your Endeca driven home page is served by a Content Collection.
Notice the priority of the Content Pages in the collection. The logged in version of the page is set at higher priority (5), than the default (10).
4) In the Content Collection for home page, there are 2 pages, one for logged in, and the other for anonymous.
The Logged In home page has a trigger based on the “loggedInUser” user segment that we created earlier:-
5) Once these settings are made, and you promote_content to production, you will get the required home pages for logged in and anonymous users.