Upham had the following question at 3DCommune, and under the premise that if one experience user is mystified then others probably are, too I’m posting it and my answer here.
Q: Well time for me to ask a question!!
Has anybody managed to understand the Selection & Skip sliders in the Selection Subpalette??!! I’ve reread pages 193 + 194 of the manual a few times tonight, and well I still don’t get it?! All this time I’ve been guessing how to place my masks on objects, adding an extra 1/2 onto my ZBrush production. I’ve never understood them from day 1, and really should climb this mountain!
Cheers in advance!
Upham 
A: There are two keys to understanding these critters. First, they only work in conjunction with Row, Col and Grid. Second, they only work on the masked sections of your object. In other words, they control what parts of your model that you wish to become UNmasked.
Selection determines the number of polygons that you wish to become DEselected (unmasked) when you click on Row, Col or Grid. Skip determines how many masked polygons to skip before running the next unmasking operation.
Take a sphere with only 30 rows and 20 columns. Turn off SH and SV so that you can see the polygons themselves. Now in order for these functions to work, the object must first be masked, so click the button to mask the entire object. Set Selection to 8 and Skip to 2. Click Row.
8+2=10, so with 30 rows of polygons, you will get a total of 3 masked areas. ZBrush starts by unmasking half of the selection value from either end – so four polygons from each pole will be unmasked. It then leaves 2 polygons masked (your Skip setting) followed by unmasking another 8 (your Selection setting). It then leaves the remaining 2 polygons masked in the very center. (You can see the polygons extra clearly by clicking outside the sphere to turn the subdivide function completely off. While you hold the mouse button down and turn the sphere, you can clearly count 4+2+8+2+8+2+4=30.)
Now reverse your values. 2 for Selection and 8 for Skip. Click the Col button. First off, the unmasked areas remain unmasked. Remember: these buttons only work on MASKED areas, because they control the polygons that will become UNMASKED. They have no effect on areas that are already unmasked. This time, though, you get 1 unmasked polygon from either end (half your Selection setting), followed by 8 polygons that are left masked, followed by another 2 unmasked polygons. 1+8+2+8+1=20 – the number of columns that your sphere is made up of.
Presto! You now have a sphere with a total of 6 masked areas, each one 8 columns long by 2 rows wide, and with a narrow space of 2 columns between them running along the equator.
As you can see, it’s actually a very powerful way to select symmetrical sets of polygons from within the masked regions of your object. You can apply it to the entire object by masking the whole thing first, or only to specific spots by painting a mask on. And of course, you can invert your mask when you’re done (which in the example would leave 6 unmasked areas).
Two special notes:
First off, if you leave Skip alone, then ZBrush will assign the same value as what you have for your Selection setting. Thus, Selection 4 and Skip 0 is the same as Selection 4 and Skip 4.
Second, it pays to start with an object that has a number of rows and/or columns that is evenly divisible by the planned sum of your Selection and Skip values. That was why I went with 30 and 20 for the sphere’s Initialize values. 8+2=10, so I guaranteed that I would get a nice even result of 3 by 2. If ZBrush can’t give you an even result, it fudges a little with the last time it performs the unmask/skip combination to even things out. It will give you the proper Skip value (the proper number of polygons to remain masked), but it will cut the final Selection value short.
Hope that makes sense! Give it a try with low poly objects with SH and SV turned off to really see it at work. Then try making a candy bar. (Hint: It’s really just an even grid with some inflated rectangles and uninflated spaces between them.)
