The grass seed should have applications rates in square feet per bag or pounds per 1000 sq. ft. for lawns ....or lbs per acre for pasture application rates. Pasture application rates will usually be much lower rates than the application rate for yards ..... so I tend to treat the two numbers as upper and lower limits for deciding what I want to put down.
For example looking at a couple different pasture grasses the seeding rate was 20lbs/acre, but looking at lawn seeding rates for same/similar grass(es) the rates are 4-8lbs per 1000 sq. ft (roughly equivalent to 160 to 320lbs per acre)
...which is why I personally tend to apply seed at a rate between the two rates, and then over seed as (or if) needed.....assuming the situation allows that.