Skill Categories are containers for Skills that can be arranged into a hierarchy and as such provide a way for you to organize and group related skills. Skill Categories are not assessable like skills are.
Skill Categories are assigned to Teams, Roles, or directly to People via Skill sets. This allows you to, for example, assign sales related skills to the sales team, or marketing related skills to marketing roles.
The Skills Directory should be grouped logically and generically according to the types and categories of skills, such that skill categories can be then assigned to teams and roles in a matrix fashion.
The Skills Directory should not be grouped by organizational function, group or role. That part comes later during assignment of skill categories to those entities. Failing to observe this will ultimately lead to duplication of skills which can cause confusion when it comes to searching and reporting on skills.
By keeping skills grouped logically according to the range and types of skills within the organization as a whole it will be possible to assign skills to teams and/or roles in a matrix fashion without the need for duplicating skills: