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What you may be confused about is size flexibility. In 2017 RDS implemented size flexibility for certain RDS platforms: https://aws.amazon.com/about-aws/whats-new/2017/10/amazon-rds-reserved-instances-offer-instance-size-flexibility/
Non-licensed RDS platforms can be size flexible. The RI purchased with the size flexible platform type, can be applied to db instances regardless of their deployment type (single- or multi-AZ) and regardless of the instance size within the same family AND generation.
However, you CANNOT modify or change any of the attributes of RDS RI itself, after you purchase it. You have to select the attributes at the beginning, when you purchase it.
Because of the size flexibility, it makes sence to just purchase RIs with the lowest instance size within the family and single-AZ deployment type. You only need to calculate how many of them you need based on the normalization factor (same normalization factors apply as with EC2): scroll to "Instance size footprint" in the docs here https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modifying.html
Above link is about EC2 RI modification, but it has a table with the normalization factors, which can be used for RDS db instances footprint calculation as well.
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