When performing a custom sort to arrange the name of the month field by sorting. sortnum, the primary field selected for sorting would be the "sortnum" field.
How to explain
This field, which presumably contains numerical values corresponding to the order of the months, becomes the key sorting criterion. Subsequently, the secondary field chosen for sorting would be the "name of the month" field.
The custom sort operation first arranges the data based on the numerical values in the "sortnum" field, establishing the desired order of the months, and then sorts the corresponding month names accordingly, aligning them with their respective numeric sort order.