Your IDoc type should have a flat structure (not too many parent-child generations). A segment may be used only once to define an IDoc type. However, since segments or segment groups can be repeated (minimum and maximum frequency of use are two of their attributes) and can be ‘optional’ only (attribute: optional segment), they may appear several times in the generated IDoc or not at all. Position the required segments as close as possible to the start of the IDoc type.
The required segments in the EDI standard should therefore correspond to those in the IDoc type. Similarly, the minimum and maximum frequency of use should be selected accordingly.
The posting module must support all the permitted IDocs in the relevant IDoc type. Similarly, the selection module must be able to create the segments in the correct sequence and hierarchy. Otherwise, the syntax check of the IDoc interface detects an error.