In 2012 in the developer workspace go to the form you want to modify like CustTable and expand the Design of it. Find the tab with the fast tabs you want to change. Within the tab find the field in the fast tab you want to change like Delivery_DlvMode. Right-click on the field and choose properties. In the property sheet for the field set the FastTabSummary to Yes, No or Auto. By default most are set to Auto which means if the field has a value for the current record the field is displayed in the fast tab. If you set the FastTabSummary property to No the field will not be displayed in the fast tab.
Changing the Fields on Fast Tabs in AX 2012
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