Here is a link to an Oracel Document about how Ultra search scores the documents:
It also has info about expanding search rules.
Ultrasearch uses Oracle Text contains(...) queries. So you can refer to http://download-east.oracle.com/docs/cd/B14117_01/text.101/b10730/ascore.htm to get information about the scoring.
For the UltraSearch docs try searching for document B10731-02 in the dos section.