If the overall consumed RU/s exceeds the maximum RU/s of the database or container, requests that exceed the maximum RU/s are throttled and return a code 429 status. Requests that result in more than 100 percent normalized utilization are throttled. Normalized utilization is defined as the maximum of the RU/s utilization across all physical partitions.
For example, your maximum throughput is 20,000 RU/s and you have two physical partitions, P_1 and P_2. Each partition is capable of scaling to 10,000 RU/s. In any given second, if P_1 has used 6,000 RUs and P_2 has used 8,000 RUs, the normalized utilization is MAX(6,000 RU / 10,000 RU, 8,000 RU / 10,000 RU) = 0.8.
Note
The Azure Cosmos DB client SDKs and data import tools (Azure Data Factory, the bulk executor library) automatically retry after a code 429 error is returned, so occasional code 429 errors aren't problematic. A sustained high number of code 429 errors might indicate that you need to increase the maximum RU/s or review your partitioning strategy to include a hot partition.