Generally speaking the default document is the document or priority in which the default document is displayed for your domain's web hosting account. This is the document that is displayed by "default" when no specific document file name is specified. For example http://www.dytek.net actually defaults to http://www.dytek.net/default.asp because the file default.asp is set as the default document.
Typically several file names are possible for your default document, by default. (default.htm, default.asp, index.htm, etc)
For customers who use our Online Account Administration (Helm) you can change and set priority for your default documents. Below is a video tutorial on how this is done.