The sort operation should create a category that combines the property identified as the sort key column in the ulPropTag member with the sort key column specified in the previous SSortOrder structure.
TABLE_SORT_COMBINE can only be used when the SSortOrder structure is being used as an entry in an SSortOrderSet structure to specify multiple sort orders for a categorized sort. TABLE_SORT_COMBINE cannot be used in the first SSortOrder structure in an SSortOrderSet structure.