Oil companies are required to report their inventory data to the EIA–it is not optional. For data supplied to the API, cooperation is voluntary. They both, however, enjoy similar response rates from companies, despite the fact that many consider the EIA to be the “official” data. Both bodies are thought to receive around a 90% response rate. https://markets.businessinsider.com/news/stocks/how-accurate-are-eia-and-api-inventory-reports-1029204634

API is voluntary reporting to a trade association. EIA is mandatory reporting.