The short answer
CALC offers two reasoning tiers: Standard and High. You switch between them with the Reasoning tier button, the gauge icon below the chat input. Both tiers and the same models are available in every plan, including Free. Standard works well for everyday engineering questions and routine tasks. Choose High for complex analyses and tasks that need more careful reasoning. If you attach a file that Standard cannot read, High answers the message, and you still pay the Standard price. If Standard can read the file, the message stays on Standard.