Need help with display sub-header in matrix

You can try to create a separate table that contains the subheader values and the corresponding classification. A workaround is to create a table with columns for "Month", "Quarter", and "Classification", where "Classification" is the subheader value and "Month" and "Quarter" are used to determine the order and grouping of the subheader values in the matrix.

You can create a relationship between it and the original data table, using the "Month" and "Quarter" columns as the keys. Then, you can add the "Classification" column to the Rows field in the matrix visual, and it should display as a subheader.