Microsoft announced in October 2022 that Azure compute Reservations purchased on or after 1 January, 2024 would no longer be able to be exchanged. There is now a grace period meaning that customers have at least until 1 July, 2024 to exchange their Reservations for Azure Dedicated Host, Azure App Services, and Reserved Instances. This is a useful article with some example scenarios: https://bit.ly/3MdGC0m.
When you purchase an Azure Reservation you choose a scope for it which defines where it may be used: within a specific Resource Group, a specific Subscription, or all Subscriptions. A Management Group is a group of Azure Subscriptions and Microsoft announce this as a new option for the scope of a Reservation.
Azure Reservations for the Azure SQL Database service became available earlier in August 2018 (see our blog post here: http://bit.ly/2KROzpp). If you want to understand more about how the reservation discount is applied automatically to running Azure SQL Databases then this is a useful article: http://bit.ly/2vX61UR.