If you deploy your data instances in a single AZ, your dedicated master instances are also deployed in the same AZ. However, if you deploy your data instances across two or three AZs, Amazon OpenSearch Service automatically distributes the dedicated master instances across three AZs. The exception to this rule occurs if a region only has two AZs or if you select an older-generation instance type for the master instances that is not available in all AZs. For more details, refer our documentation.