[Empada]

Well... I remember something about it, but I don't remember were.
IIRC it is:
8-9 skills you learned a long time ago, and don't use regulary (topics from School)
10-11 hobby skills and background skill
12-13 professional skill (most professionals, not specialized. Ex. a doctor)
14-15 expert level ( most specilaized and high risk professions. Ex. a Surgeon)
16+ master (life risk and high specialized professions. Ex. a Brain Surgeon)

the list probably have the 20+ and 25+ but I don't remember
also, these values considerate the +3 or +4 bonus for routine tasks, so a professional with only 12 would roll 15-16 for normal duty.

these modifiers are on Basic set for sure. the list maybe are on the Skill chapter

edit: look on basic set page 171 - Meaning of Skill Levels
and page 345 for task dificult