Appendix A. Color Modes and Bit Depth
A short explanation why and when some blend modes are not available in Photoshop.
The blend modes in Photoshop were generally written for 8-bit operation in the RGB color space. As a result, several of them do not work in other color spaces or at higher bit depths. Refer to the PDF specification, ISO 32000-1, to see the complete publicly available blend mode calculations.
Of particular note are the limitations on 32-bit depth, which is used for High Dynamic Range (HDR) images. These are typically composite images made up of very large ranges of brightness. 32-bit mode precludes some blending modes because ...