
Some of Quicksilver’s search options are only available by clicking the black gear icon at the top-right of the drop-down list of search results. The exact effects of these options can be unclear - some are dependant on two options being selected, and QS neglects to show which options are currently being used! (The latter issue will hopefully be fixed in a coming version.) The first menu in the screenshot controls how search results are ordered in the drop-down list. Its submenu controls where to search when typing, and whether the best match rises to the top of the list or stays where it is.
The options work on the contents of Objects. Objects include the Shelf, the Clipboard, folders and text files. Typically, contents are obtained by → into an Object in the first pane. Also, when invoking QS, typing into pane 1 searches the entire Catalog (as defined in Preferences>Catalog) and the results returned are treated as the contents of an Object. Or, the contents of a ‘Search Object’ for the benefit of this post.
Quicksilver has the ability to search and sort an Object’s contents (whether they’re in the Catalog or not).

Here’s the menu items and their effects on an Object’s contents:
I recommend using ‘Sort by Score’ and ‘Filter Results’. With these options, QS searches the folder or text file I’m browsing, and the kind of things I’m looking for are always near the top of the drop-down list. Most of the unwanted ‘noise’ around the returned results is removed.
‘Sort by Name’ is occasionally handy if I need to see where a group of similarly named results are within an alphabetical list to decide which one I want. ‘Filter Catalog’ can be ‘set’ anytime by pressing escape while the drop-down list is visible. ‘Snap to Best’ could be useful if the top hit needs to be viewed within an unfiltered alphabetical list. Personally, I don’t see the need for a Finder-style list of results in Quicksilver.