The ATM method is a dual method where old stock is treated accordingly with the FIFO method, and new inventory is subject to the average cost method. This accounting method has been invented to meet the new Austrian tax requirements for 2023.
Example: A purchases 1 BTC for 100 EUR on 01.01.2021 and another BTC for 300 EUR on 12.07.2021. Following the inventory rules, the first BTC purchased is accounted for using the FIFO method. If A sells the first BTC on 01.01.2023 for 300 EUR and then buys 2 more BTC three months later for 300 EUR each, A now owns 3 BTC. With the transactions provided, the new cost basis for each coin is 300 EUR.
Please note that this method is still in beta testing and results may vary in the future due to further improvements.