Hello

Candidates who are studying in Class 12th and will appear for the board examination in 2019 are eligible to apply for the NEET UG exam. Such candidates will be considered for admission only if they qualify the exam and fulfill the eligibility criteria.

Such candidates have to select the Qualifying Examination Code- 01. In the field asking for Year of Passing/appearing Class 12th/qualifying examination, they will have to select 2019 and the next field which ask for
% of marks obtained in Class 12th/qualifying examination is to be left blank.

Coming to your second question, if you are reappearing for Class 12th examination in 2019 then you are eligible to fill the form and will be considered as appearing candidate. You will have to select qualifying examination Code 01 and other fields are to be filled as I mentioned above.

Hope this helps.
Let me know if you have any other query.
Good Luck!