A Skill is not "Scoped" by an applies to class, meaning it does not have an applies to class attribute. You would scope it (determine the extent of reuse) using the Ruleset. If it is a skill which is potentially used throughout the organization, it should belong to the Organization Ruleset. If it is only applicable to an application, use the FW ruleset.
Many moons ago my colleagues and I leveraged user Skills to create a more robust GetNextWork function that allowed for weighted value calculations as to the most important work item for a given operator. Not a best practice, but a way to further micro-manage the list of "best work" per user requirements.