Hi and welcome! TC and the modes of Ni, Ti, SS, refer to the metal the actual coil, the wire itself is made from, Nickel, Titanium, and Stainless Steel. Ni is what was, and is still, a popular material for use in factory coils. Ti has mainly been used by folks who build their own coils, and SS is gaining in popularity for both builders and factory coils.
For safety reasons, both Ni and Ti are limited in use only to TC. This is because of concerns that should those coils made of those metals get too hot, they might theoretically emit nasty stuff you really don't want to breath in. SS is popular as it is fine in both TC and power/wattage mode, so less hassle remembering which coil is in which tank.
There are 3 settings because each metal changes resistance at different rates when current is applied, and that's what the mod monitors to reach and maintain a given temperature. Using the wrong option with the wrong wire will provide a less than great
vaping experience.