There are 2 outputs for the function actionrwd.rwd (see the documentation here: https://docs.lokad.com/reference/abc/actionrwd.reward/):

- Demand: it estimates the probability distribution of the demand that is not covered yet by existing stock and that happens within the coverage timespan. This distribution may have non-zero probabilities in the negative values: this simply means that even if no order is placed, the entire demand might still be covered, with stock remaining at the end of the coverage timespan.
- Holding Time: estimates, for each extra unit that could be purchased, the average time it will stay in stock before being sold.

From your description, I assume you are talking of the Demand output. To complement the explanation of the documentation, you can also see this demand output as the probability to need x additional units in stock to satisfy the future demand. If x is negative, you get the probability of reaching the end of the timespan with abs(x) units in stock without making any additional order.

I hope this helps,