The 'Custom Columns' sheet is created by default. It is formatted with the usual two/three sections.
QDV7 populates this table with free numerical field whose attribute "Is part of cost or selling price" is to Belongs to ... Refer to How to Create Free Columns with Custom Behavior.
▪those whose attribute "Is part of cost or selling price" is set to Belongs to ...
▪those whose attribute "Show in totals" is set to Sum (which requires to have a formula).
The Extra Costs to be Allocated to WBS as per Allocating Extra Costs To The WBS Tasks.belong to the last category.
In case at least one of the fields has been declared as belonging to cost or selling price, a total shows in the corresponding column.
NOTE If this sheet is missing, switch to Edit mode and check the A2 cell as described in How to Hide/Unhide a Sheet.