In the rest of this document, "8bpp" is short for "8 bits per pixel", which means a 256-color mode. Similarly, 15bpp refers to 32768 colors, 16bpp to 65536 colors , 24bpp to a "packed" 16 million color mode, and 32bpp to a "sparse" 16 million color mode (at 32bpp, only 24 of the 32 bits are actually used, hence the "sparse").
15bpp is only used here to differentiate it from 16bpp, but they are both normally referred to as 16bpp. 15bpp is actually 16bpp with a 5-5-5 color weight (wasting one bit per pixel), while 16bpp is, well, 16bpp, with 5-6-5 color weight.