For a long time, the standard was a 6×4 configuration in which both rear tandem axles receive power. Trucks came with 6×4 regardless of the jobs they were designed to do.
However, some fleets and drivers have lately been switching to 6×2 configurations in which only one of the rear axles receives power. The other is a “tag” or “dead” axle whose only function is to bear weight.
Spec’ing one of these options over the other comes down to decisions about applications, driving surfaces, weather conditions, fuel efficiency, load weights and driver preference.