The DEFINESCORE operator enables you to define how the score for a term or phrase is to be calculated. The DEFINEMERGE operator defines how to merge scores of child elements of AND and OR operators. You can also use the alternative scoring template with SDATA to affect the final scoring of the document.
"Alternative and User-defined Scoring" for information about the alternative scoring template
Oracle Text Reference to learn more about the syntax of DEFINESCORE and DEFINEMERGE