Allows to change the blending color space and/or the luminosity softmask blending color space (if defined in the PDF document).
When Adobe Acrobat interprets transparent objects and attempts to create a preview of the flattened artwork, it needs to do so using a single color space for all the items involved in the transparency group. This color space is called the blending color space.
The blending color space for soft masks (masks with soft transitions) is called the luminosity softmask blending color space. It is usually gray.