What feature allows it to "guess" your rating? Is it automatic (after a number of games it guesses your rating), or do you just set one manually and upgrade it

What feature allows it to "guess" your rating? Is it automatic (after a number of games it guesses your rating), or do you just set one manually and upgrade it yourself over time? I did not find any mention of such feature on https://www.shredderchess.com/ https://www.shredderchess.com/