Atom ID - Modifying lammps

Dear Lammps users,

I am trying to extend Lammps to a new force-system for spherical particles in a box. I just need to access the atom ID. My question is in what variables are the local and global atom IDs stored?

I understand that it is very basic, but I am missing out on something in my search.

Thanks and regards,

Sandesh Kamath