C# and SQL - sub select from a table parameter

Question

Below is a code snippet for passing a table as a parameter to a query that can be used in Sql Server 2008. I'm confused though about the "SELECT id.custid FROM @custids id". Why does it use id.custid and @custids id...?

private static void datatable_example() 
{

   string [] custids = {"ALFKI", "BONAP", "CACTU", "FRANK"};

   DataTable custid_list = new DataTable();
   custid_list.Columns.Add("custid", typeof(String));

   foreach (string custid in custids) {
      DataRow dr = custid_list.NewRow();
      dr["custid"] = custid;
      custid_list.Rows.Add(dr);
   }

   using(SqlConnection cn = setup_connection())
   {
      using(SqlCommand cmd = cn.CreateCommand()) 
      {

         cmd.CommandText =
           @"SELECT C.CustomerID, C.CompanyName
             FROM   Northwind.dbo.Customers C
             WHERE  C.CustomerID IN (SELECT id.custid FROM @custids id)";
         cmd.CommandType = CommandType.Text;

         cmd.Parameters.Add("@custids", SqlDbType.Structured);
         cmd.Parameters["@custids"].Direction = ParameterDirection.Input;
         cmd.Parameters["@custids"].TypeName = "custid_list_tbltype";
         cmd.Parameters["@custids"].Value = custid_list;

         using (SqlDataAdapter da = new SqlDataAdapter(cmd))
         using (DataSet        ds = new DataSet()) {
            da.Fill(ds);
            PrintDataSet(ds);
         }
      }
   }

Answer 1

'id' in the subquery is just an alias.

The same way you have 'C' as an alias in the main FROM clause and use 'C.CustomerID'.

The subquery could just as well have been

 SELECT custid
 FROM @custids