This format is an extension of BI_BITFIELDS where a fourth bitmask is used to define bits in the pixel which correspond to an alpha channel. When displayed on top of other images, RGBA pixels are blended with the background pixel according to the value of this alpha component.
For example, a 32bpp RGBA image would likely use the top 8 bits of each u_int32 to store the alpha component (the unused byte in normal 32bpp RGB). In this case, the masks reported would be:
giving 256 levels of blending per pixel (8 bits of alpha data).
In general, the masks used for this format are passed using a means other than a BITMAPINFOHEADER (for example, in DirectDraw, the DDPIXELFORMAT structure contains fields specifically for R,G,B and Alpha masks) but I have also heard that it is acceptable to append 4 u_int32s to the end of the BITMAPINFOHEADER structure containing the mask information.
Child formats: RGBT