N1 Nismo option (available to Nissan Motorsport account holders) rear beam axel was manufactured with a setting of 0 degrees of rear toe-in.
This aided turn in at corner entry over the standard specification (JN15/FN15/N15) rear beam axel that was set with toe in.
Toe in is generally used on the rear to stabilise the car at corner entry by causing the outside loaded wheel to be angled into the direction of the corner, it also stabilises the car in a straigh line under both acceleration and braking.
This was often added along with the much sought after Nismo option body mounted rear sway bar / stabiliser kit
...And the need for the narrower ET45 rear offset on the 0deg rear beam, is because the stub axel mounting plates that dictate the amount of toe, were altered at the forward most part of the beam, this caused the mount plate, and therefore axel and wheel to sit somewhat further out, this widened the track width. But as track width has an adverse effect on other aspects of handling, the ET45 offset was used to bring the (overall) track width back to that of the same measurement as the ET40 with the standard toe in setting.
My Nismo rear beam also has the trailing arms mounted a little further in when directly compared with a standard beam.