Prerequisites
You must have completed the required steps in
Extending an IDoc Type .
It can occur that you want to fill table fields which already have an equivalent in the basic type but which are not supplied in the standard system. You therefore do not need to extend the basic type, but rather only the function module that supplies the application fields. In this case, skip the procedure.
Procedure

The name of your extension should be no more than 8 characters if you wish to use the extension to communicate with partners who are using SAP releases prior to 4.0. You can only use extension names with more than 8 characters from Release 4.0 onwards.
The basic type is displayed as a tree structure. See the color legend (Utilities ® Color legend).

You should bear the following factors in mind when dealing with extensions:

In the example call your extension Z1ORDERS. Append child segment Z1TEST1 to reference segment E1EDP19 in basic type ORDERS01. The child segment can be used only once.