"Many countries provide mobile boxes at the request of voters who may be elderly, ill, or otherwise unable to visit a polling station. Usually, the mobile boxes are taken on their rounds by at least two polling officials, ideally representing different political interests where applicable. Applying all polling-station controls to mobile ballot boxes is not possible. Voters using mobile ballot boxes may also not have all the privacy afforded by a polling booth."
OSCE (ODIHR): Election Observation Handbook (Fifth Edition), p. 57

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Election Observation Handbook (Fifth Edition)