Page 5 informs us all the dice in each 'roll' must be re-rolled, unless the rule granting said re-rolls also provides permission to pick and chose which dice to re-roll and which to keep. Just keep in mind that the important words used in that last sentence was in each 'roll' because it is possible to roll multiple 'rolls' simultaneously. There are ways to roll multiple individual 'rolls' at the same time, such as the fast dice method. In these cases we still treat each separate 'roll' as if it was done one at a time, applying all the rules that would take effect, even though it is being rolled at the same time as other dice. This could give you the ability to select which dice to re-roll and which one to keep even with the requirements to roll 'all' the dice, because they are a separate roll. Though to be honest, I am a little hard press to think on a situation where you would be rolling multiple 'rolls.' Fast dice is applied when multiple rolls have the same rules and can therefore be applied simultaneously without changing the final results. The rules which force re-rolls are often trigged on only the dice which produce a negative effect, granting permission to keep the positive results. So I do advise you to take a good look at the rule you are using to gain the ability to re-roll as the answer has to be in there.
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