Message Code in IDOC

Hi Gurus,

Can you please explain whether Message Code and Message Function can be entered randomly while creating partner profile in IDOC or we can use some specific only ?

I mean to say:-

Suppose I want to create one partner profile with Message Type ORDERS

Now when creating an Outbound Parameters can I enter any name ( say XY or YZ or ABC etc.) in Message Code and Message Function field and put similar words when creating Inbound Parameters of another partner profile to link sender and receiver?

If it is not, then can you please let me know how can I get the list of Message Codes and which one has to be used where?

Thanks in advance

Amit