With limited exceptions, there is no requirement that posted vacancies be open to members of the public.
exception---As a result of the Veterans Employment Opportunities Act of 1998 (5 U.S.C. 3304), any preference-eligible as well as any veteran who has been honorably discharged from the armed forces after 3 or more years of active service can apply for vacant positions posted under merit promotion procedures whenever the hiring agency is accepting applications from individuals outside its own workforce.
merit promotion procedures---neither the Rule of Three that limits selections to the top three interested candidates nor veterans preference rules apply to merit promotion postings.
Applicants are not ranked on the basis of their numerical scores. Instead, merit promotion certificates contain the names of all candidates (usually listed alphabetically) determined to be among the best qualified, and the managers are free to select any of those candidates for the job.