Please go to https://www.crossref.org/ and register to become a member. Once on the above link, scroll down a little and you will see a yellow box that says Crossref DOI display. Click on it to read the guidelines on same. It will take you to the following link https://www.crossref.org/display-guidelines/ which will give you a lot of information and hopefully answer some of your questions. Failing that, they have a 'How can we help you? search box at the top of each page. Hope you find this helpful

DOI is a unique key allotted to any object to locate precisely and correctly on the internet. It is useful in cross referencing, most of the journals prefers to include DOI of  referred manuscript to be mentioned in the bibliography in your research paper. it facilitate the readers to locate those reference material on the internet. DOI is a combination of alphanumeric keys, it is different and unique for each object. for more information and details you may like to refer the link. any article published in such journal will have its own unique DOI given by the journal. hope this will be of some use to you.

https://www.doi.org/hb.html

Thank you so much Anna crouch sir and Anand

Hi Saroj,

I have just glanced through the handbook recommended by Anand and I think you will probably find the answer to your questions in chapter 9 of that hand book. Good luck.