If you find the need to alter Idoc field values to make an easy match with the recieving system - this can be done as a customizing thing... (extending to ABAP logic if you like). This is about a setup which I´ve not seen on many Idoc implementations, however it´s very powerful and it´s good to know about it´s existance.
Just to situate yourself: an outbound Idoc that is pure standard SAP is created, e.g. the CREMAS message type. You want to change the field value for the E1LFA1M-MSGFN field, using a translation table. Thus 004 becomes X04 and 005 should become XX4.
Set up a conversion rule Transaction BD62 should be used to link a conversion rule to a segment. Transaction BD79 lists the fields that can be set (manipulated) with the conversion rule. Select a field and double click on it to find out what conversion setting are available. It´s possible to set a field value to a constant value, a variabele value, a conversion can be done, a general rule can be applied, special conversion routines (like ALPHA) can be used. Lots of possibilities to change the field contents for a given segment field.
Link the conversion rule to the Idoc Which is done per sender/receiver combination along with a segment type (read: segment name) (check table TBD22 for available examples / message types).
The actual conversion takes place just before the (communication) Idoc is created, thus the setup allows changing the Idoc contents of a standard SAP Idoc without ABAP logic. Try it out, it can be a low impact life-saver ! However: if you do use it, make sure the development team learns about this, to avoid spending a lot of time on phantom problems.....