In the second quantization spin operator, what are Pauli spin vector indices?

[univox360]

If you look up the second quantization spin operator, you'll notice that there are two indices on the pauli vector for two possible spins. The operator sums over these two indices.

Since the pauli vector is an unchanging quantity what do these indices physically correspond to?

 

[univox360]

For example what's the difference between sigma up up and sigma down up?