When registering to vote, you usually register for the county you live in. You cannot vote outside of that county unless you change your county or opt for mail in voting. This is because different counties have different representatives in both their state and the federal government. In short, it’s ment to prevent people from voting multiple times or screwing with a different county’s vote. I didn’t know how to opt for mail in and didn’t know my local representatives and what they supported, so I never reregistered which meant I could not vote in the general election.