when do we ceate custom messagae type apart for custom idocs type

Hi all.

Can u pls let me know when do we create custom message type in ALE/IDOCS. what I know is we can create a custom message when we create custom idoc type & when we have reduced idoc type.Pls let me what are other cases in which I need to create a custom message type.Thanks in advance.

Kind regards,

sami.