You can send the id you use to identify users on your backend. This will allow backend integrations for one to one messaging or based on segments created on your backend.
There are two functions available, setUserId and setExternalId. As of the release of user profiles, in most cases we recommend using setUserId. However, if user profiles are not enabled for your project (profiles are enabled by default, so only older projects might not have them enabled) you should use setExternalId.
You can check if profiles are enabled for your project by clicking "data" from the left menu, and seeing if there is a profile section.
Profiles not enabled:
xtremepush('set', 'user_id', CUSTOMER_ID);
IDs can be used later for targeting users in the segmentation engine or via API.