Its a decal, skid plates and shocks on the option. It might have the all terrain tires instead of the regular tires, although both the same size. Also you couldn't option for the 20' wheels on some models, 18"s were the max for the z71.(Skid plates are the only real bonus, and you can add them w/o having the z71 package, for about 1/2 the cost (I think its $135 for skid option)(z71 was about $250ish if I remember correctly)
If you option for the z71 appearance package, on 2011- 2014 hd trucks or some of the 1/2 tons prior to the '14s, it was a color matched grille and bumper assembly. (I believe the appearance option was around $900-1250)
Millertime- Sabre Handler