Which model of mill Jennie? The different models work differently and give different results. So which one are you looking at?

For example the Victorio/Roma/Squeezo types

Victorio

Roma

using the standard screen gets all the seeds and skin, core pieces, etc. The stuff that remains after processing the lightly pre-cooked whole tomatoes is a solid compact mass of the stuff you don't want. So solid and dry that it can be difficult to clean. I do recommend staying away from the vacuum clamp ones however. The vacuum just doesn't hold as well as the actual clamps.

The Foley food mills

Foley food mill types

have a more coarse screen so some seeds and a little skin can get through. Some models come with a set of interchangeable screens.

The old fashioned food mills like

old-style-food-mill

get it all and keep out all but the smallest seeds but also has more waste so you don't get it all and is much more work.

The so-called "tomato press" types like have no advantages other than cheap price IMO as many seeds and skins pieces get through.

Tomato-Press

So the Victorio, Roma and Squeezo using the standard screen are the two most commonly recommended models and you'll find several past discussions here about the pros and cons of them. You will get all the juice and I'd estimate 95+% of the pulp. With no further cooking down and stirred well it makes the ideal tomato juice consistency for jarring. For sauce some further cooking down is needed unless you pre-roast the fruit or let it settle out overnight in the fridge and then skim off the water that floats on top.

Hope this helps.

Dave