Hi Olivier, in order to understand the symbols you must purchase the publication of ISO226. This MATLAB function only implements the mathematics described there. But to summarize, dozens of loudness tests have been performed over the last ~80 years and this function fits a curve to all of these tables. If you read the paper many of your questions will be answered.
It's not precised in this script what is it ?about af, Lu, Tf, and Ln, , what are the symbol represented by af, Lu, Tf, and Ln? Where the vales come from (arbitrary or not) Tf af, Lu, Tf, and Ln : what is it ?