GDR-DU mean General Distribution Release (Dynamic Update)
Sadly from TechNet no resource seem wrote for that. From the information I have it's file kept for upgrading from a build to another, to prevent a fresh system re-install (like build 1511 to 1607). (not for normal updating).
GDRDU just means that when you install Windows 10
1607, it will automatically download and inject the
update as part of the upgrade process, as long as
dynamic update is enabled (which is the default).
It's a mechanism to get fixes in place without
refreshing the entire OS image.
Windows 10 GDR-DU: GDR = General Distribution Release, DU=Dynamic
Update. These updates are only used by the current GDR build of
Windows 10 when it is trying to perform a Dynamic Update (see item 5
above).
Item 5: Windows 10 Dynamic Update: This includes only updates to the
setup process that occurs when one build of Windows 10 is trying to
update to a new build of Windows 10 (i.e. 1511 to 1607).
A more generic description for GDR-DU.
By default, all components on Windows systems start on the GDR branch
following each major release. When you install updates from Windows
Update for a GDR component, it gets upgraded with the GDR version.
When you install a specific Hotfix, the files and components in the
Hotfix package are migrated to the LDR branch. At this point, that
particular component is marked as a LDR component. If you install a
newer Update over this component, the Windows servicing technology
will automatically install the appropriate latest version from the LDR
branch for you. This is possible because each Update package ships
with both the GDR and LDR versions of the component.
Once a component is marked as a LDR component, the only way to move
back to the GDR branch is to uninstall all Hotfixes for that
component, or move to the next available service pack.
GDR-DU mean General Distribution Release (Dynamic Update)
Sadly from TechNet no resource seem wrote for that. From the information I have it's file kept for upgrading from a build to another, to prevent a fresh system re-install (like build 1511 to 1607). (not for normal updating).
A description for GDR-DU from a Microsoft source there, official (but on an archived mailing list). (https://marc.info/?l=patchmanagement&m=147085158401556&w=2)
See there for a quick info, not official, but still point to the same answer than the Microsoft's source: http://www.urtech.ca/2016/12/solved-what-do-the-windows-10-product-names-mean-in-wsus/
A more generic description for GDR-DU.
Quoted text from https://blogs.msdn.microsoft.com/ntdebugging/2008/10/21/windows-hotfixes-and-updates-how-do-they-work/
The question after is, why you can select GDR-DU alone without the Dynamic Update too.