Explanation
Choose a partition key that has a wide range of values and access patterns that are evenly spread across logical partitions. This helps spread the data and the activity in your container across the set of logical partitions, so that resources for data storage and throughput can be distributed across the logical partitions.
Choose a partition key that spreads the workload evenly across all partitions and evenly over time. Your choice of partition key should balance the need for efficient partition queries and transactions against the goal of distributing items across multiple partitions to achieve scalability.
Candidates for partition keys might include properties that appear frequently as a filter in your queries. Queries can be efficiently routed by including the partition key in the filter predicate.
References:
https://docs.microsoft.com/en-us/azure/cosmos-db/partitioning-overview#choose-partitionkey