Hello Student
Marks in a spcific subject of 12 class examination you can appear for all subject improvement exam for more better result in the subject. But the marks you have got in that subject is  not satisfactory and you can get more marks if you appear for improvement exam of that specific subject.. The marks of the subject is added with the main marksheet they there will be a fresh marksheet and certificates for your 12th class examination. Hope this will help.
Hope this helps.