What this means in practice
Credits cover the underlying usage instead: review the balance and the cost of the operation you run if you want to understand a charge.
Practical guidance
The subscription and the activity are two different parts of the cost. Your plan gives you access to the agent and its capabilities; credits are consumed by the underlying work performed during a run. This is why two requests made under the same plan can use different amounts: their model work and other billable operations may differ.
Before a task that may involve several steps, check the credit balance and the usage information shown for the run. Break a broad request into a clear outcome and review the proposed work when you need tighter control over usage. If a charge is unexpected, compare the run details with the credit balance rather than looking for a second agent subscription.