Hi , 

Master IDOC : This idoc collects all the required data into 
single IDOC and this is temporarily stored in memory. 

Communication IDOC: there can be n number of communication 
IDOCs generated. This comes into picture after the Master 
IDOC is validated from the distribution model . And this 
IDOC is stored in database table . you can see this IDOC 
from we02 tcode. 

Best Regards, 
Suresh .

Inbound process strats when application data is creatred
inthe  database.

Based on the given input outbound program trigers ,reads the
application data from the database& generates the master idoc.

Master idoc is the data inthe internal table. master idoc
data will not save any where in sap.

comm idoc is the physical idoc which is reciever specific.

1 master &communication idoc

5 master idocs and the number of communication idocs is 
based on the number of recipient systems which is mention 
in the customer distribution model of that message..

master idoc: in the outbond process all the data in sap r/3 
is retreived via outbond program.after retrieving, the data 
is arranged in segments in heirarichal order in an internal 
table.this internal table which has segments and data is 
known as master idoc.

communication idocs : this completely depends upon the 
distribution model which has list of senders and 
receivers.so if receivers are not attached to senders in 
distribution model than we have zero communication idocs.so 
communication idoc is simply the data which gives the link 
between the senders and receivers.

if u send 5 records it means u r sending 5 master idocs and 
the number of communication idocs depend upon the number of 
receivers u have in the distribution model.

Master IDoc(s) : The internal table(s) having Segments and
Master Data is known as Master IDoc. The number of Master
IDocs generated are depend upon the number of records you send.

Communication IDoc(s) : The number of Communication IDocs
generated are depend upon the number of Receivers you have
in the Distribution Model.