Why are transparent sleeves not allowed in Riftbound?

Ruling: Transparent sleeves are not allowed because they provide none of the protective uniformity of opaque sleeves and none of the full visibility that judges get from unsleeved cards, making card marking easier and harder to detect.

Nuances:

• Transparent sleeves are not allowed for main deck, rune deck, or battlefields (though enforcement on battlefields may vary as marking them provides no competitive advantage)
• Tokens can use transparent sleeves as they are not game objects
• Players can use different colored opaque sleeves for main deck vs rune deck vs battlefield to distinguish them
• Double or triple sleeving is allowed as long as at least one layer has an opaque back; transparent inner or outer sleeves are acceptable if there is an opaque layer
• Cards can be marked by tucking them left or right in sleeves, and manufacturing cut issues on cards may make them recognizable through transparent sleeves
• In best of 1 formats, transparent sleeves on battlefields could allow players to see which battlefield is which during random selection