Your API version controls the API and webhook behavior you see (e.g. what properties you see in responses, what parameters you’re permitted to send in requests, etc.). Your version gets set the first time you make an API request. When we change any of our APIs in a backwards-incompatible way, we release a new dated version, but to avoid breaking your code, we don’t change your version until you're ready to upgrade.
Each of Clearbit's APIs is versioned separately. You can set the API version on a per-request basis, or globally in your dashboard. It's a good idea to keep the API version you're using up to date - we're often pushing shiny new data attributes.