For the legacy tracker doc, check our guide here.
This page and the ones that follow describe how to use the REST API to implement the Brevo Tracker features. The JS implementation is simpler and supports more capabilities. For example:
You can combine both tracking technologies. For example, use JS on the client side to identify and track user behaviour on your website, and use REST on the server side to track orders.
The REST implementation requires a key, shown as client_key below, to authenticate. Get it from your Brevo account.