To be added somewhere on the documentation (maybe the result page it self). Also open to discussion for modification.
Relevance score is computed buy search api full text. In short it allows to search the filed below with some weight for some fields (all at 1.0 or 2.0 for now) .

Example: Searching "test" http://biii.eu/mevislab-xmarkershortestpath-module-dijsktra-shortest-path returns a score of 7:
because:

• Additional keywords contains twice "Shortest", score = 2*1.0

• Title contains twice shortest, score= 2*2.0

• Documentation link title contains also test score = 1*1.0
  TOTAL relevance score = 7
  Idem with http://biii.eu/test-java3d scoring:

• Additional keywords contains tester, score = 1*1.0

• Title contains Test, score= 1*2.0

• Documentation link and downlad page contains also test score = 2*1.0
  TOTAL relevance score = 5

IMPORTANT: WHILE the search should also be performed in synonyms for now it is not (see LSFM) , new issue for that