Competencies can be broad categories, down to specific child elements of the categories. For instance, "data structures" may be a general competency, whereas "hashmap" may be a more specific competency within the "data structure" category.
A person is thought to have a competency or mastery of a skill if:
For most subjects, an indivdual's competency exists on some continuum, and hence why we have an ELO system, and potentially other evaluation mechanisms such as badges that can be used to demonstrate the competency.
As an example, as specific competencies within a broader category become more advanced, or more obscure, it is less likely that an individual will have competency in the specific example. In this scenario, for an individual, it would show that you had strong ELO in the broader competency, but not so much in some of the specifics.