"The version of the AIFS available at the time of writing is trained on a subset of the ERA5 reanalysis for 1979–2018 and fine-tuned on operational IFS data from 2019 to 2020. Pressure level fields and surface fields are used for training, together with forcing data, like time of the year and insolation."
https://www.ecmwf.int/en/newsletter/178/news/aifs-new-ecmwf-forecasting-system