"""I am also curious to know how error message can be displayed through incompletion log since I was getting warning message and was able to proceed further but while saving, the document shows in incomplete status""
You can check based on the Flag-Incomplete messages in VOV8
(If it is deactive-then you will get the option about incompletion).
Also to change the Message status from W to E--Try with OVAH (else at se91 at coding level)
Note:Incompletion procedure will not work based on customer--if business wants based on customer--then you need to go with exits.
Very Imp Note: Incompletion log will always check whether some value to be maintained in the respective field--but it does not validate(other than f4 options for some fields)
Phanikumar