You may apply to any job; however, you may not be eligible for the job if you do not fall into one of the required categories. If you are not eligible, your application may not be considered. For example, any U.S. Citizen is eligible to apply to positions that are “Open to the Public”. Look for the "This job is open to" section in the job announcement to see who is eligible to apply.
The job announcement will list who is eligible by using one or many USAJOBS hiring paths. Visit the USAJOBS hiring paths page for more information: https://www.usajobs.gov/Help/working-in-government/unique-hiring-paths/.