This is the datamodule where all the datasets for the report are located, and the published properties defined. Normally it will be the same datamodule that has the TFlexCelReport component, so it's assigned automatically and you don't have to care about it. 

Really, the datamodule doesn't need to be a TDataModule descendant, but any TComponent, as long as it holds the necessary DataSets. That said, for any medium-complex project, it will probably be one TDatamodule dedicated exclusively to the reports. 

Known issue: You can not set this property to point to another DataModule at design time. This is an issue with the Delphi IDE, and the only workaround is assigning this property on your code. 

TFlexCelReport Properties