What does the "D" mean in SQL syntax?

Question

Piotr Kononow's post List all indexes in SQL Server database contains an example of SQL query to list all indexes in a SQL Server database. And we have confirmed in testing that the example works indeed.

To fully understand the query, we have a question regarding the below cross apply section, please also refer to the full query included in the details at the end.

  cross apply (
    ...
  ) D (column_names)

Our Question:

What does it mean by the D and D (column_names)?

We highly appreciate any hints and suggestions.

Details: the full sample SQL query:

--Reference:
--  https://dataedo.com/kb/query/sql-server/list-all-indexes-in-the-database

select
    i.[name] as [index_name]
  , substring(column_names, 1, len(column_names)-1) as [columns]
  , case
      when i.[type] = 1 then 'Clustered index'
      when i.[type] = 2 then 'Nonclustered unique index'
      when i.[type] = 3 then 'XML index'
      when i.[type] = 4 then 'Spatial index'
      when i.[type] = 5 then 'Clustered columnstore index'
      when i.[type] = 6 then 'Nonclustered columnstore index'
      when i.[type] = 7 then 'Nonclustered hash index'
      else 'ERROR'
    end as [index_type]
  , case
      when i.is_unique = 1 then 'Unique'
      else 'Not unique'
    end as [unique]
  , '['+schema_name(t.schema_id)+'].['+t.[name]+']' as [table_view]
  , case
      when t.[type] = 'U' then 'Table'
      when t.[type] = 'V' then 'View'
    end as [object_type]
from
  sys.objects t
  inner join sys.indexes i
    on t.object_id = i.object_id
  cross apply (
    select
      col.[name]+', '
    from
      sys.index_columns ic
      inner join sys.columns col
        on ic.object_id = col.object_id and ic.column_id = col.column_id
    where
      ic.object_id = t.object_id
      and ic.index_id = i.index_id
    order by key_ordinal
    for xml path ('')
  ) D (column_names)
where
  t.is_ms_shipped <> 1
  and index_id > 0
order by i.[name]

;

Answer 1

What does it mean by the D and D (column_names)?

D represent as a table alias while the column_names is for column alias for col.[name]+', '.

The column_names was used on SELECT list: substring(column_names, 1, len(column_names)-1) as [columns]

To learn more about apply operator I'd recommend to read these links: link 1 and link 2.