Here is a brief description of ADM and ADMX. First, let’s describe what an ADM and ADMX file is.
ADM File: The ADM file was used with Group Policy before Windows Server 2008 was released. The ADM file contains all the settings that are found under Administrative Templates. Each time a new Group Policy is created, the settings for the Group Policy is stored in the SysVol share. The ADM file is also stored with the Group Policy setting.
This means that Group Policy using the ADM does not scale well as it makes the SysVol share very large. Also once a Group Policy is created it is linked to the one ADM file. The ADM file only supports one language so if multiple administrators were working on the same Group Policy one language would need to be agreed between all administrators.
ADMX File: The ADMX file replaces the old ADM file. It was first introduced in Windows Server 2008, however, if you download the latest Group Policy Management software you can use the ADMX files in early Windows Servers. ADMX is an XML based format making it easy to edit. ADM is an in-house format so it is not as easy to work with as XML files are. The format is made up of two parts.
The ADMX file defines the Group Policy settings. The ADML file contains the language to be used with the file. This means the ADMX file can easily be used with any language assuming an ADML file for the language exists. Both ADM and ADMX output the same files so regardless of which format is used, they will be compatible with old and new clients.
For more information about creating the central store for Group Policy Administrative Templates, see the following article
For a more advanced tool to manage Group Policy, see the following article on Microsoft Desktop Optimisation Pack (MDOP), https://techdirectarchive.com/2020/04/27/microsoft-desktop-optimization-pack-mdop-at-a-glance-agpm-mbam-app-v-dart-med-v-and-ue-v/
ADM vs ADMX: Here is a brief comparison between an ADM and ADMX file and how an ADM file can be migrated.
ADMX files use less space in the SysVol. You can choose to store the ADMX and ADML files in the SysVol. The difference between ADM and ADMX files is that if you choose to store the files in the SysVol, each ADMX file only needs to be stored once where the ADM files need to be stored for each Group Policy that is created.
- ADM are text-based documents and can be created or modified via a text Editor. - ADMX are XML based document. With an XML document, this can be viewed.
Does your system still use ADM? certainly not sure, but take a look on how to have them converted.
Note: ADM cannot be used in some Server Editions but can be converted to ADMX using ADMX migrator. This can be found in the Microsoft download center.
What is ADMX Migrator: ADMX Migrator is a snap-in for the Microsoft Management Console (MMC) that simplifies the process of converting your existing Group Policy ADM Templates to the new ADMX format and provides a graphical user interface for creating and editing Administrative templates. https://www.microsoft.com/en-us/download/details.aspx?id=15058
ADMX Migrator, which is created and supported by FullArmor, enables you to convert ADM files to the ADMX format and take advantage of the additional capabilities that it provides. The new XML-based format includes multilanguage support, an optional centralized datastore, and version control capabilities.