Event Source

Event source is where an event was sent from: a web page, a mobile app, a backend service or a bulk import of historical data.

Most libraries stamp this onto every event as a property, and teams usually add a more specific label of their own. It matters because one logical action can arrive through two pipelines, and without a source you cannot tell real growth from double counting. Reliability is the second reason: browser-side events can be blocked and server-side events cannot, so the two will never agree exactly. The common mistake is stacking sources into one chart and reading their gap as an instrumentation bug, when the gap is usually the blocking rate or a difference in coverage between the paths.