This is a complex question. I will break this down into multiple parts to make this easier to understand.
To start off your mod uses your historical data to calculate your current year experience rating factor.
The historical data used, is the following:
Three out of the last four years of data used to calculate your current experience rating factor.
If your rating period is 01/01/2021- the 01/01/2020 data is not used, the 01/01/2019, 01/01/2018 and the 01/01/2017 data is.
Part 2 will bread down what data is used.