there is no explicit criteria from what it seems.
the only thing that is certain for now is they ain' giving LHB to trains that have slip coaches
also, they seem to avoid popular dailies, which in a way is good as Gen coaches usually get reduced in order to account for EOG coaches. popular daililes require numerous rakes as many of them are long distance trains, and they prefer to make 3 trains LHB with 3 rakes rather than using it all on a single train, so popular trains like GT, TN,...
more... kerala aren't gonna get converted soon