What is the CPQ API and when would you use the Calculate or Read APIs?

The CPQ REST API (available at /services/apexrest/SBQQ/ServiceRouter) allows external systems and Apex code to interact with CPQ quote data without going through the UI. Key APIs:

1. Read API (reader=SBQQ.QuoteAPI.Reader): Returns a JSON representation of a quote including all quote lines, options, and pricing data. Used to read the current state of a quote programmatically — for integrations that need to send quote data to an ERP or CPQ reporting system.
2. Calculate API (saver=SBQQ.QuoteAPI.Saver): Takes a JSON quote payload, runs the CPQ pricing calculator, and saves the updated quote. Used when building custom configurators, integrations, or batch re-pricing jobs. The flow is: Read → modify the JSON → Calculate/Save.
3. Product Search API: Queries available products with pricing for a given quote context. Use cases: recalculating prices in bulk after a price book change, integrating with a third-party e-commerce or CPQ tool, and automating quote creation from an external system (e.g., a partner portal).