I have added feilds in Addtitional Data B screen o SO,Now How will I Update

Hi All,

I have added feilds in Addtitional Data B screen on SO,

on a Particular Sales Document Type(ZTAL) I have to Enable one feild called Leasing Duration. in all other cases it has to be disabled or invisible.

How do i do it?

Also please let me know ?

How will i update the table VBAK once these feilds are enterd in the SO Addtitional Data B screen by the user and clicks save....

Should i code my logic for that in PBO .... of SAPMV45A -8309

Or should i go for a User Exit or a BADI

Please Answer!