An applicant with a service-related disability gains certain protections by disclosing that disability on a federal job application, particularly if the job carries a veteran's preference. I have heard that some federal jobs allow applicants to voluntarily disclose a disability on the application, as some jobs apparently give a preference to disabled workers who are returning to the workforce, but that information is not disclosed to the person doing the hiring unless it becomes relevant to the hiring decision and application of the preference.