Aka-dora are red-colored fives that replace a normal five in one or more suits: 0m, 0p and 0s. Their number depends on the ruleset. Each red tile is worth one bonus han, like a dora, but through the tile's identity rather than an indicator.
Aka-dora are optional. The Tenhou profile taught here uses one red five in each suit, while WRC 2025 uses none. Each red five in an ordinary winning hand adds one han and stacks with dora and uradora. A red five remains an ordinary five for every hand shape.
A central five fits tanyao and many shapes, but keeping it still depends on shanten, ukeire and danger. Aka-dora are not yaku, so a hand made only of red fives and dora cannot win. If an opponent calls a red five, that bonus becomes part of their exposed hand if they later win with a yaku.