Skill Level

The Skill Level is one of the ways that the final Ability Rating of a Skill is determined. 
When a skill is boosted in any way, it is always the Skill Level that is being modified. The skill's Ability Rating is then re-computed based on the Skill Level, the Five Attributes, and any applicable penalties from injuries. 
Likewise, when a skill is being checked, for example to determine an Adventure's requirements, the Skill Level is always the relevant number.