Hello, there is no additional biology subject in the NEET. 6th Subject or additional subject is a vocational subject along with the other 5 compulsory subjects like Physics, Mathematics, etc. In this system, if the student fails in any one of his compulsory subjects, then the marks of the additonal subject are considered and the final percentage is calculated by replacing the marks obtained in the failed subject by the marks obtained in the additonal subject. However, earlier the students who had additional biology that is their 6th subject as biology were not considered as eligible to apply for the NEET examination. This led to a chaos among the NEET aspiring Student community. The parents and the students protested against this order which led to change in the order where now the students who have their 6th subject as biology are allowed to apply for the NEET examination, 2019. Thus, it is now clear that students who have biology as an additional subject can apply for the NEET 2019 without any complications. 

All the best!