@Martin Hornáček here is the blog on partition key you can refer to
https://parveensingh.com/cosmos-db-parition-keys/
I find this article very good as it explains physical partitions in relation to how cosmos DB scales
https://learn.microsoft.com/en-us/azure/cosmos-db/scaling-provisioned-throughput-best-practices
Also the following video Better understanding database throughput and partitioning in Azure Cosmos DB - Episode 38 - YouTube) In the video, PM Deborah Chen, walks through what happens during partition splits.
Regards
Geetha
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