Symbian has a very elegant, document-centric design: For almost all files the system knows which application is able to view and process which file. In a file manager, a file is shown with the icon of the corresponding application, and a tap on the icon automatically starts that application and tells it to load the file.
Windows has a similar system that relies on file name extensions, e.g. .doc for Microsoft Word files. This system is very brittle however: Change the extension of a Word file from .doc to .htm, and Windows will hand over the file to the default Internet browser which won't be able to do something sensible with it.
Symbian is much more robust because it does not rely on extensions, but on so-called recognizers, something like "mini programs" which look into the files and determine their types based on the first few dozens of bytes or even less. With the file type known, Symbian can then check the database of installed programs and start one which is able to handle files of the given type.
The MoStacks package includes such a recognizer.
The trouble is however that a recognizer installs into a very delicate spot of the system, and as such it must be trustworthy, and that means it must be signed with a proper certificate. Right now (November 2008) the recognizer could not yet be signed, which makes installing it difficult, because you have to self-sign the .sis file at SymbianSigned.com before it will install.
In order to be able to release MoStacks on UIQ3 at all despite this problem, as an alternative MoStacks is able to work along the lines of the inferior Windows system and "recognize" stacks on their .MoStacks extension. It uses this alternative method with the check box Use recognizer? in the Settings Dialog unchecked (which is the default state of that setting).
In this mode MoStacks can only work with stacks that are contained in one of the 3 possible stack folders, according to the settings, or in sub-folders of those folders, and are therefore listed on the start view.