TCR is Temperature Coefficient Ratio or how much the resistance of a particular wire changes resistance in relation to the temperature of the wire.
There are TCR tables for different wire types on Steam Engine and various places on the net.
Once you get the value close you can fine tune it a bit if you want.
The fixed TI, NI, SS TC modes in mods have a TCR value programmed in for each of the wire types.
TCR is because one size does not fit all. Different types of SS wire with different TCR values and such as that.
I do not yet use TC but do pretty much understand how it works.