Document IDs facilitate document management in SharePoint.
For more information about Document IDs, see the Microsoft article Enable and configure unique Document IDs.
Migrating Document IDs to an on-premises version of SharePoint
Document IDs are not supported when you migrate to an on-premises version of SharePoint. Even though ShareGate Migrate can sometimes migrate them, there is no guarantee it will work.
Mapping Document IDs
You cannot map Document IDs when you migrate content in Insane mode.
When you migrate in Insane Mode, SharePoint will always use the source's Document IDs at the destination (if the source is empty, the destination value will remain empty).
You can use Normal mode to map your Document IDs.
Migrating Document IDs from on-premises SharePoint to SharePoint Online in Microsoft 365
Document IDs are not fully supported when migrating from an on-premises SharePoint source (such as SharePoint 2016) to Microsoft 365.
In some cases, Document IDs may appear to carry over during the initial migration, but Microsoft 365 will reassign new IDs when documents are moved within the environment.
This happens because Document IDs in on-premises versions of SharePoint lack the flags required to fully support their preservation in Microsoft 365.
New Document IDs can also be assigned when copying documents from certain lists and libraries.
Note: If you migrate from a Microsoft 365 source that contains Document IDs previously preserved from an on-premises migration, you may experience the same behavior.