Implementaion of polarisation in itp file

Dear Gromacs Users,
I am doing simulations using Gromacs 20201.4 software. I want to add polarisation in itp file. According to the gromacs manual, the format of polarisation in itp file is:
[ polarization ]
; Atom i j type alpha
1 2 1 0.001
But I am confused about what is meant by an atom, i, j and how the parameters are taken for these. Can anyone please help me with this?
Thank you very much in advance for your kind help.

Regards
Hema Teherpuria