CommonInfoForm is a Django ModelForm that is customised to allow it to work with CommonInfoMixin. CommonInfoMixin overrides the model instance’s save method and adds a required user argument, so it can keep its user_modified field up to date with any and all changes made to the instance. This prevents it being used with a standard ModelForm, as the form’s save method will attempt to call the model instance’s save method without the user argument.
CommonInfoForm overrides the form’s save method so that it does call the instance’s save method with a user argument. It also overrides the form’s constructor to accept a user keyword argument on instantiation, so it has a known user to use in the save method. The user argument of the constructor is only required when the form is bound - as this is the only time save would be called - but it is always accepted. And subclasses of CommonInfoForm may well choose to make use of the user argument in other ways, when the form is bound or not. Best practice would be to always instantiate CommonInfoForms with a user keyword argument.