Skills sets start with low level sets and then proceed to ever increasing levels of specialisation. Each low level gives a broad understanding, but specialisation is required to become very good at a task. The intention is to allow the system to cope with a stone age character living in a space age world without creating system induced anomalies. Being able to develop a skill is based on a set of prerequisites as are the levels within that set of skills. For basic skills these prerequisites are only abilities, but for higher level skills other skills may form the prerequisites for the skill.
Skills can be categories into skill sets.
This could be rear as the curriculum of learning required to develop skills, but also represents the set of skills required to be considered a skilled person at a task. Thus a religious cleric would have certain mandatory and optional skills to develop to be considered a cleric. The line between skills and characteristics is quite blury. The general approach is that skills require composites of characteristics and or other skills as prerequisites. Skills External emotional abilities relate to the interaction of the character with the surrounding environment.