Hi Samy,
This form is build using some grouping of data. The InventDimId field is used here. If you have e.g. Site and warehouse enabled and you specified to see only on hand on Site level, the InventDimId is not used known as it is a summation of multiple records with different InventDimId's. So linking on the InventDimId of the InventSum datasource is not possible.