I have a picklist field on opportunity, which contains some values for example: A, B, C, D
The user must be able to update A or C during record edit if he tries to edit values other than A and C validation rule fires. But when trigger fires it should be able to update any of the values validation rule should not be fired.
I tried creating one flag on opportunity but this method will allow both system and user to edit the field.
So please provide any suggestions on this??