TEXCOORD 0 1 2 3 for cg shader scripting?

I was wondering what the differences were between TEXCOORD0 TEXCOORD1 TEXCOORD2 and TEXCOORD3 in cg shader scripting. What exactly is each one used for and when do you use it? Thanks in advance

TEXCOORD0 is the first texture coordinate channel, TEXCOORD1 is the second and so on. Unity currently only supports 2 channels(I think 5.0 may support more).