MAIL-IN VOTING: ELECTION INTEGRITY SAFEGUARDS
Ballot style codes
Most ballots have proprietary style codes, such as timing marks, code channels, or QR codes. If the codes are not recognized by the voting machines, the ballot is rejected, and an election official manually reviews it.
Ballot paper specifications
Most ballots are printed on a specific type
of paper. If the paper does not match the specifications in length, paper weight, or opacity, the voting machine will reject the ballot
Ballot watermarks
Some ballots have watermarks indicating that the ballot is authentic. Watermarks are specific to each election and confirm the ballot was printed by an approved printing authority
Sauce: https://www.cisa.gov/sites/default/files/publications/mail-in-voting-election-integrity-safeguards_508.pdf