what is difference between code 2 and3 in neet application form

Code 02 is for those students who have completed their 10+2 from any other State except for Karnataka. Code 03 is for those students who have completed their 10+2 from Karnataka.

Code 02 says: The Higher/Senior Secondary Examination or the Indian School Certificate Examination which is equivalent to 10+2 Higher/Senior Secondary Examination after a period of 12 years study, the last two years of such study comprising of Physics, Chemistry, Biology/Bio-technology (which shall include practical tests in these subjects) and Mathematics or any other elective subject with English at a level not less than the core course for English as prescribed by the National Council of Educational Research and Training after introduction of the 10+2+3 educational structure as recommended by the National Committee on Education.

Code 03 says: The Intermediate/Pre-degree Examination in Science of an Indian University/Board or other recognised examining body with Physics, Chemistry, Biology /Biotechnology (which shall include practical test in these subjects) and also English as a compulsory subject.

Hello !!
code 2 is for the candidates who have completed their 12th that is equivalent to 10+2 higher /senior secondary examination(CBSE).Here the candidates will have to undergo a period of pre-professional training before admission to the medical colleges.

Code 3 is for the candidates who have completed their intermediate from a board or recognised university in PCB.For more details about codes in NEET ,just go through the link provided below:

https://medicine.careers360.com/articles/neet-eligibility-criteria

hope that helped you,if any queries you can mention in the comment session.

Good luck!!