@AzureUser-9588 Thank you for your query!!!
As mentioned here The reservation discount applies to the resource usage matching the attributes you select when you buy the reservation. Attributes include the scope where the matching VMs, SQL databases, Azure Cosmos DB, or other resources run. For example, if you want a reservation discount for four Standard D2 virtual machines in the West US region, then select the subscription where the VMs are running.
If the virtual machines are running in different subscriptions within your enrollment/account, then select the scope as shared. Shared scope allows the reservation discount to be applied across subscriptions. You can change the scope after you buy a reservation. For more information, see Manage Azure Reservations. You can also use the management group scope. It applies the reservation discount to the matching resource in the list of subscriptions that are a part of both the management group and billing scope.
You can also split a reservation into two reservations to apply some of the instances you bought to another subscription.
So it is possible to share Reservation across Subscription. This change in directory of RI does not check for if the tenant is in the same EA or account. While transferring an RI, the billing aspect changes as well. this would just be changing the directory of the RI and billing remains as it is hence you can move it across tenant.
Hope it helps!!!
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