-c[i] ? What s the exact logic for it?

Why we are doing -c[i]while order_of_key in PBDS

as (a[i] - b[i]) > (b[j] - a[j]) in manipulation, how (b[j]- a[j]) can be -c[i]?

Please explain the reason behind why (b[j] - a[j]) < -c[i]