located hereprobability :: double (optional): A value in range [0, 1]. Item or fluid is only given with this probability; otherwise no product is produced. Has no effect when amount is specified.
but when looking at angels bio processing amount and probability are commonly used together successfully(not sure if the resulting product is intended, but not the point). It gives the [amount] of product with [probability] chance. In-game it shows that it will produce [probability]*[amount] of product, which is definitely not correct as it is not guaranteed to produce that, and it is never able to produce that amount (unless probability = 1).