Thank you for using Engine3!
Engine3 is not using classic OCR methods but uses large language models (LLM) for the OCR task. The compute and server costs for Engine3 are much higher, therefore we need to have different limits for Engine1/2 vs 3.
The monthly PRO/PRO PDF plans include 30,000 conversions for Engine3 in addition to the 300,000 for Engine1/2.
If you need more than 30,000 Engine3 conversions per month, the best way is to get a second OCR subscription, and then use both API keys in your API calls. This will give you 60,000 conversions total (or 90,000 for 3 subscriptions/API keys, etc) for Engine3.
All this is in addition to the 300,000, 600,000 or 900,000 conversions that you have for Engine1/2.
For more details on how to add more api keys see this forum post.