Please keep in mind that the Custom field is just a placeholder for the groups/categories you want to use and not the actual categories. If you have a field called "Dimension" where the possible dimensions are "Dimension A" and "Dimension B", you are just now creating the field Dimension but not the 2 possible values for this field. The two possible dimensions "Dimension A" and "Dimension B" don't need to be created beforehand, they will be created automatically when you create an indicator belonging to that category.