Reflections of stars, for example in a lake or in windows, do no move in the same direction and at the same speed as the stars in the sky. Because of this, Starry Landscape Stacker cannot align stars in the sky at the same time that aligns stars in a reflection. If you have an image with stars in both the sky and a reflection, first process the images like you normally would, being sure to erase any red dots in the reflection and that the blue mask is only over the sky and not over the reflection. After you have saved the resulting image (possibly adding “-sky” in the file name), create a new mask for the reflection. Usually, I do this by selecting “Adjust Stars” in the workflow buttons, selecting “No dots”, and then possibly painting in some dots in the reflection before selecting “Find Sky” painting the mask over the stars in the reflection. You want to have a mask that covers all the stars in the reflection and no visible objects that are not moving with the stars (e.g., logs or rocks in a lake, the lake shore). Then select “Align and Save” a second time. This time add “-reflection” in the file name. Now combine the sky and reflection images in your favorite image editor, showing the sky from your sky image and the reflection from your reflection image.
There is a video tutorial on processing images with refections here.