A game can have two spare cells. Each cell contains a card, or it may be empty. The card in a spare cell is available to all players to combine with cards from your hand or from the table.
It is not allowed to play a card from a spare cell before you have played a card from your hand.
Only deal to the spares at the start of the game
With this option set, at the start of the game a card is dealt to each spare cell. After you played the card from a cell, that cell remains empty for the rest of the game.
Deal each turn to empty spares
With this option set, at the start of the game a card is dealt to each spare cell like with the previous option. But if a card is played from a spare cell, then at the start of the next player's turn, a new card is dealt from the stock to that cell.
You may play onto the spares
Like before, cards are dealt to the spare cells at the start of the game. But now, if you have played a card from a spare cell, you may play another card from the table or from your hand onto the empty cell. If you don't play to an empty cell, a card is dealt to that cell when the next player's turn starts.
It is not allowed to win the game by playing to a spare cell.
Games with this rule
Of the built-in games in RRRummy, only Accelleration and Rozzy use spare cells.
No other Rummy type game, including Tile Rummy, uses spare cells this way.