The global vs local trend removal slide bar in ArcMap 9.3.1 is used to determine how locally the trend will be removed. 100% global will remove trend using global polynomial interpolation, and anything else will use local polynomial interpolation. Read about these two methods in our help, and feel free to ask questions about anything you have trouble understanding. Note that in ArcMap 10, you can actually see the trend removal parameters, so you have more control over how the trend is removed. As for how you decide how locally to remove the trend, there are a couple of things to watch. First, if you remove the trend too locally, you may find that the semivariogram on the next page is near flat. That means that you removed too much trend. You want to remove just enough in order to still fit a good semivariogram. Second, you can go by crossvalidation statistics, which I'll talk about below.
The biggest things to look for in crossvalidation is that the average standard error should be similar to the root-mean-square, and the root-mean-square standardized should be close to one. If you find several models that have these properties, choose the one with the lowest root-mean-square. You can also see if the mean standardized is close to 0, but if the other statistics are good, this one is usually good too.