Reg. sc/st certificate

Hi,

   All the public sector undertaking companies requesting the 'prescribed form of sc/st certificates' separately in a paper during the written test or during interview, though we have the life term community certificate, why?

When i approached the thasildar , he asked income certificate attached, vao--->RI---->thasildar----> verification

Is it the process to get that, Plz let me know how to get that certificate ?

thanks,

suraj