We first draft the arm opening, then we draft a sleeve to fit it.
When all goes well, the sleeve fits perfectly, and this option is not relevant. However, if/when the sleeve does not perfectly fit the arm opening, we need to adapt it.
Since the shape and height of the sleevecap are more important than the width, we tend to prefer adjusting the width to fit the sleevecap.
To avoid the sleeve getting too narrow, we only adapt a portion of the sleeve width. This option allows you to control how much of the sleeve width is guaranteed. Increasing this option will allow us less of the width to play with to fit the sleeve. This way, the resulting sleeve will be closer to the theoretical sleeve width, and we will instead modify (more of) the sleevecap height to fit the sleeve.