When you refer to fields in the row area of a marketing document, the column number is displayed in addition to the item number. In the example here you can see that the ItemCode field in the sales order document has item number 38 and column number 1.
You will notice that the item and column number for a field is the same across similar document types, for example, the ItemCode has item number 38 and column number 1 in all sales and purchasing documents.
Why is this useful? You can use the item and column number instead of the table and field name in a query. Queries using the item and column number are often used in approval procedures, when we want to use the same query for multiple document types. Obviously, a query that used the table and field name would not work across multiple document types.