I've seen two different fatality rates discussed for diseases when I try to look at papers, the infection fatality rate and the case fatality rate, and in the case of Covid-19, there's recent work about trying to determine the infection fatality rate and the case fatality rate.

However, when it comes to the seasonal flu, I normally just see things along the lines of "the fatality rate is .1%". What I don't understand is which sort of fatality rate that is and how it's actually being calculated. Is that .1% of people who get infected with a flu virus, or .1% of people who have noticeable symptoms of the flu? Does this vary for different seasonal flu strains, or does the number of cases cause the variance in flu deaths each year?

In short, what are the best numbers for the seasonal flu that Covid-19 fatality rates should be compared with once the Covid-19 studies are considered sufficiently complete (if we're not already at that point for some regions), and how are those values determined for the flu?