Thank you for posting your query on Microsoft Q&A, from above description I could understand that you are looking to migrate from On-Prem to Entra Domain Services (formerly Azure AD DS) and are looking for matching custom attributes datatypes.
Please do correct me if this is not the case by responding in the comments section.
Microsoft Entra Domain Services integrates with your existing Microsoft Entra tenant. This integration lets users sign in to services and applications connected to the managed domain using their existing credentials. You can also use existing groups and user accounts to secure access to resources. These features provide a smoother lift-and-shift of on-premises resources to Azure. As you have an existing on-premises AD DS environment, you can synchronize user account information to provide a consistent identity for users. To learn more, see How objects and credentials are synchronized in a managed domain.
As per Extension attributes Microsoft Entra ID offers a set of 15 extension attributes with predefined names on the user and device resources. These properties were initially custom attributes provided in on-premises Active Directory (AD) and Microsoft Exchange. However, they can now be used for more than syncing on-premises AD and Microsoft Exchange data to Microsoft Entra ID through Microsoft Graph.
Microsoft Entra Domain Services can synchronize the following types of extensions from Microsoft Entra ID, so you can also use apps that depend on custom attributes with Domain Services:
- onPremisesExtensionAttributes are a set of 15 attributes that can store extended user string attributes.
- Directory extensions allow the schema extension of specific directory objects, such as users and groups, with strongly typed attributes through registration with an application in the tenant.
Both types of extensions can be configured by using Microsoft Entra Connect for users who are managed on-premises, or Microsoft Graph APIs for cloud-only users.
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