Pro, Max and Team include model usage, so you do not need a key. Past a limit, usage credits keep the agent working at standard metered rates, until you turn them off or reach a spend limit you set. Credits come with Pro, Max and Team, and are useful when you do not want work to stop at a limit.
Enterprise has no included allowance. Its model usage is billed at standard metered rates from the first request, within spend limits that admins set for each user and the organization, unless the organization uses its own model key.
With included usage, credits or metered usage, prompts and responses pass through Emolysis's metering service on the way to the model provider. That service is designed not to store or log their content, and to record token counts for billing. The Free plan stays bring your own key, and every plan accepts your own key. Read model keys and credits and what happens at a usage limit.