Bottoming tap holes are useful in situations where the full length of the hole is needed for threading, typically to maximize thread engagement in limited space. In SolidWorks, creating a bottoming tap hole involves specifying the thread to go all the way to the bottom of the hole, unlike a standard tapped hole where the thread depth is less ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.