your broker or fund company preset a default cost basis method for you. That default method probably won’t give the best tax results when you sell shares. What you may not know is, you can change the default method to the one you want to use.
But know that once shares are sold, there’s no way to go back and retroactively make changes. It’s best to set your method of choice now before you start selling off shares. This way you won’t unknowingly lock yourself into a bad decision.
