Hi Stanzin,

Paper codes refer to the codes assigned to disciplines or subjects. You may apply for any of the 24 papers for Graduate Aptitude test in Engineering. You are expected to ytake u those papers relevan to your qualifying degree. In order to register there is also a provision to apply without giving paper code. ( Ref. https://www.digialm.com/EForms/configuredHtml/844/51671/Registration.html )

Please refer to below link to understand more about paper codes:

https://engineering.careers360.com/articles/gate-exam-pattern