This format can be thought of as a simple extension to the basic 16bpp and 32bpp flavours of BI_RGB. RGBT uses the most significant bit of the pixel (unused in RGB 16bpp and 32bpp) to indicate transparency. If the bit is set, the pixel is visible, otherwise it is transparent. You can also think of this as a version of RGBA where the alpha channel comprises a single bit.
Based on: RGBA