Updated for Archicad 25.
Pen sets let you change the appearance of output at the very last moment - when the drawing, based on the view, is headed out the door via the layout. The colors you see while working on the model can be completely different from the published output, and they should be.
Color (Working) Pen Sets
As described in the Pens post, building material cut fills are made with three rows of pens, which allow us to control the visibility of the cut fill patterns.
When working on the model, you should usually be viewing it in color.
Color No Hatch hides the fine hatches and shows the coarse ones as black. The cut and uncut pens (from the typical rows) are in color.
Color Wall Section shows all the hatches as black. The cut and uncut pens are in color.
Color/Black Wall Section shows all the hatches black, as well as the cut and uncut element pens (the typical rows).
Black & White (Output) Pen Sets
For output, the Bmat backgrounds are turned uniform color (usually gray), and the hatches will either match the background (to hide them) or be black (to show them).
B/W PDF Output is the default set. The Bmat backgrounds are gray and the fine hatches are hidden.
B/W PDF 1/4" Floor Plan is for typical plans. The Bmat backgrounds are gray and the fine hatches are hidden. The 14-weight pens, used for cut lines, are heavier.
B/W PDF 1/8" Floor Plan is for small scale plans. The Bmat backgrounds are gray and the fine hatches are hidden. The 14-weight pens are lighter than the 1/4" set.
B/W PDF Framing Plan is for framing plans. The Bmat backgrounds are white and the cut pens match the 1/4" plans.
B/W PDF Wall Section is for wall sections. Most Bmat backgrounds are white and all the hatch pens are black. The backgrounds of the GWB and sheathing Bmats are gray.
Pen Sets in the Workflow
There is a pen set dedicated to the model, and a pen set dedicated to the layout book. If you go to Attributes -> Pens and Colors and choose a pen set, that set will be applied to the model or the layouts, depending on what kind of window is active at the time. The model set can be freely changed and it doesn't affect output. (Mostly, work in color.) The layout book should be set to one of the B/W sets and there is rarely a reason to change it.
Pen sets meet the output at the Drawing element. All drawings need a pen set defined for them. The output sets are applied to drawing elements in the layouts according to the drawing type. The placed drawings in the template all use the correct pen set. To help with this, it's convenient to have a Pen Set column in the Drawing Manager.
A note on 3D vectorial drawings: Vectorial drawings from the 3D window, such as the top view used for the site plan, ignore the pen weights of the pen set assigned to the drawing. These line weights come from the 3D window setting saved with the view, as set in 3D Styles --> Contours.