GetLabels return labels that identifies given application additional labels are used only when creating object if you are creating something use additional=true if you need labels to filter component than use additional=false.

ApplicationLabel is label key that is used to group all object that belong to one application It should be save to use just this label to filter application.

AdditionalApplicationLabels additional labels that are applied to all objects belonging to one application Those labels are not used for filtering or grouping, they are used just when creating and they are mend to be used by other tools.