You perform an axisymmetric analysis on a bolt-and-nut assembly to plot the stress results on the threads. By performing a 2D simplification analysis, you save time because 2D models require fewer mesh elements and simpler contact conditions compared to 3D models.
To see the model, open install_dir\samples\whatsnew\2dsimplification\bolt_nut.sldasm.
The bolt head and the clamped members are not modeled. To apply 2D simplification, the threads are modeled as separate rings of material and not as a continuous helix (this is a simplification, as helical threads are not axisymmetric). To model the tightening of the joint or sliding of the bolt, you still need to use a 3D model.