Read about the various cures.
Sodium nitrates are used in long curing processes like dried salamis where the sausages may cure for several weeks. Nitrate breaks down into nitrite over time.
Sodium nitrite is used for shorter duration curing or when the sausage is going to be at a temperature where bad bugs thrive for any amont of time. Like when lightly smoking a sausage for flavor. Polish, keilbasa, cooked salamis, etc need something with sodium nitrite in it (prague powder 1, tender quick, 'pink' salt)
Fresh sausage that is going to be frozen right away doesn't require sodium nitrites. Fresh breakfast sausage, patti sausages, and the like are good with just salt in the mix.
Cure no 1 contains sodium nitrite. Cure no. 2 contains sodium nitrate.
Pure sodium nitrite will kill you dead in very small amounts. Cure no 1(prague #1, tender quick, etc) has a very small amount of sodium nitrite in it. The rest is salt.