they are listed:
SOURCE2_RGB_ARB 0x8582
SOURCE2_ALPHA_ARB 0x858A
OPERAND2_RGB_ARB 0x8592
OPERAND2_ALPHA_ARB 0x859A
operand2 is used for combiner mode GL_INTERPOLATE_ARB. as the name says, this mode interpolates between operand0 and operand1. operand2 is the weight.
you can use it as usual. an use would be reflection mapping with a per pixel reflection map. you can use the alpha of your diffuse map to say, how much the material is reflecting the environment at one point. use GL_INTERPOLATE_ARB to interpolate between the rgb values of the diffusemap (base texture) and the environment map. operand2 is set to the alpha value of the base texture.