The documentation surrounding AWS Aurora reserved instances is a little confusing. One of its main selling points is seamless Multi-AZ replication at the storage level; however, when you purchase reserved instances the Multi-AZ option is fixed at "No". Does this mean you can't run read replicas in another AZ if you're using reserved instances? Will you be charged the on-demand rate for your read replicas?
The most information I could find was a document from Amazon stating that despite the field being fixed at "No", all Aurora clusters will be created Multi-AZ automatically by default. While this is true, it doesn't answer the question as to whether the read replica it creates will be covered by your "No multi-AZ" reserved instance purchase.
After experimenting, and an email to AWS, I can confirm that a reserved instance purchase does cover read replicas in other AZs (provided you purchase enough instances!) despite the purchase form seeming to indicate otherwise. It'd be a lot easier for them to fix the Mult-AZ option at "Yes"!