SERP Index is a database of Google search engine results pages that we collect and update across all supported locations. We use this data to provide SERP-related data in DataForSEO Labs API and DataForSEO SERPs Databases. On the contrary, we do not use SERP Index in SERP API because its data is collected and handed out in real-time.
Keyword Index is a database of keywords and related data collected, regularly updated, and maintained by DataForSEO. This database serves as a kernel of the DataForSEO Labs API and Keywords Database. At the same time, similarly to the Keywords Data API, Keyword Index is collected using Google Ads services.