Box 1: Core (SQL)
Core(SQL) API stores data in document format. It offers the best end-to-end experience as we have full control over the interface, service, and the SDK client libraries. SQL API supports analytics and offers performance isolation between operational and analytical workloads.
Box 2: item id -
item id is a unique identifier and is suitable for the partition key.
Reference:
https://docs.microsoft.com/en-us/azure/cosmos-db/choose-api
https://docs.microsoft.com/en-us/azure/cosmos-db/partitioning-overview