The experiment key, often referred to as the "tracking key" in the context of GrowthBook, is used to identify experiments in the SDK. It is part of the configuration when setting up feature flag experiments. The tracking key can be set when you are creating or configuring an experiment rule in the GrowthBook UI. By default, the tracking key is the same as the feature name, but it can be any string you choose.
Docs on tracking keys:
https://docs.growthbook.io/feature-flag-experiments#running-experiments-with-feature-flags