Each state has its own rules on whether someone who have suffered a felony conviction is eligible to vote.
In California, persons who have been convicted of a felony can vote as long as they are neither in prison nor on parole.
When someone is sentenced to jail or prison in federal court cases, the sentence usually includes a period of "supervised release" to begin upon release from custody. I've wondered whether someone on federal supervised release is barred from voting in California because supervised release is somewhat equivalent--though not the same--to being on parole.
Happily, the California Secretary of State has now made clear [here] that persons on federal supervised release can vote (as long as they are also not on state parole).