Update backup-support-matrix.md

Added details on cross-subscription backup scenario, also covered at https://learn.microsoft.com/en-us/azure/backup/backup-azure-move-recovery-services-vault#use-azure-portal-to-move-recovery-services-vault-to-a-different-subscription

Author: juanandmsft

articles/backup/backup-support-matrix.md

@@ -35,7 +35,7 @@ The following table describes the features of Recovery Services vaults:
 **Machines in a vault** | Up to 2000 datasources across all workloads (like Azure VMs, SQL Server VM, MABS Servers, and so on) can be protected in a single vault.<br><br>Up to 1,000 Azure VMs in a single vault.<br/><br/> Up to 50 MABS servers can be registered in a single vault.
 **Data sources** | Maximum size of an individual [data source](./backup-azure-backup-faq.yml#how-is-the-data-source-size-determined-) is 54,400 GB. This limit doesn't apply to Azure VM backups. No limits apply to the total amount of data you can back up to the vault.
 **Backups to vault** | **Azure VMs:** Once a day.<br/><br/>**Machines protected by DPM/MABS:** Twice a day.<br/><br/> **Machines backed up directly by using the MARS agent:** Three times a day.
-**Backups between vaults** | Backup is within a region.<br/><br/> You need a vault in every Azure region that contains VMs you want to back up. You can't back up to a different region.
+**Backups between vaults** | Backup is within a region and subscription.<br/><br/> You need a vault in every Azure region and subscription that contains VMs you want to back up. You can't back up to a different region. Cross subscription backup (RS vault and protected VMs are in different subscriptions) isn't a supported scenario.
 **Move vaults** | You can [move vaults](./backup-azure-move-recovery-services-vault.md) across subscriptions or between resource groups in the same subscription. However, moving vaults across regions isn't supported.
 **Move data between vaults** | Moving backed-up data between vaults isn't supported.
 **Modify vault storage type** | You can modify the storage replication type (either geo-redundant storage or locally redundant storage) for a vault before backups are stored. After backups begin in the vault, the replication type can't be modified.