Hi @utrautma . It’s basically so that you can include extra data in the requests sent from the JS tracker to the Matomo instance. For example, several of our plugins use it to send additional fields/data with each tracking request using something like _paq.push(['setCustomData', 'mykey', 'myvalue']); which means that requests sent from that tracker going forward will include data: {"mykey":"myvalue"} as a property on the request.