IDoc (for intermediate document) is a standard data 
structure for electronic data interchange (EDI) between 
application programs written for the popular SAP business 
system or between an SAP application and an external 
program. IDocs serve as the vehicle for data transfer in 
SAP's Application Link Enabling (ALE) system.
Idoc structure is 
1 Data record : This data record can be one or more .
2 control record : This control record shd be one and it 
contain the info like who is the sender , who is the 
receiver , type of message etc.
3 Status record: This status records can also be one one or 
more basing upon the the stages the idoc is tranfered.
these status records are 1 to 75.
1 to 49 are reserved for outbound and 50 to 75 are reserved 
for outbound.