Two Shastra Sahams?

Different sources give different calculations for a saham of the same name. TN means that its from Tajika Neelakantha and so probably the better one to use. 

Where do you see this option?

It's not an option.  In the Saham table in the Varshaphala chart screen there are two Shastra sahams listed, one with TN and one without.  In the Varshaphal class manual there is only one calculation for the Shastra saham, so I'm guessing that is the TN one.