Touchstone is a file format that provides a standardized way for programs to output S parameters (S11, S21) so they can be read by other programs. The extension can be .s1p, .s2p etc.
There are many programs that can import S parameters. I have a RigExpert antenna analyzer and I often import NanoVNA Touchstone files with AntScope 2 so that I can compare the measurements made with the two instruments. Antscope 2 is a free download and you can use it without a RigExpert. It has nice plotting and scaling options.
Roger
Roger Need