Facebook Conversion API (CAPI) primarily evolved from server-side API lets advertisers send web events from their servers to the Facebook server. CAPI data flows into Facebook and is treated just like pixel events allowing advertisers to measure, track and optimize their conversions.
A visitor enters into the browser and loads the website, the pixel fires an event at the browser level. Based on the visitors’ activity i.e., purchased, viewed pricing page, added to cart, subscribed, etc. the browser sends updates to Facebook and is passed down to Ads manager. This helps advertisers learn how many leads are generated, purchase made, added to cart, viewed pricing page, etc.
In Conversion API, a visitor enters into the website after clicking a Facebook ad. Now Facebook assigns a unique id for that user and tracks the visitor via the id. Whenever the visitor performs an action like added to cart, subscribe to a newsletter, etc., The server updates the action performed by the visitor and sends it to Facebook.
Right now, both browser and Conversion API sends data to Facebook. If both the events are received by Facebook, then it will deduplicate(exclude the additional event) the event and use the browser event alone.
Also, you could implement Facebook Conversions API for various e- commerce platforms like WooCommerce, BigCommerce, Shopify etc.