An insert is a liner - different terms for the same thing.

That is what is confusing me; when I look up this product, it lists an insert, with the lights, baffles, etc., a blower, and a liner, that looks like it is simply a metal piece that lines the inside of the hood and fits up against the insert. This is the piece I am not sure I need. If the insert is only 38", won't there be a lot of space around it where the wood is exposed?

It may depend on the brand/model what they call an insert and liner. You should call Zephyr directly and see what they say. An insert inside a hood is smaller than the hood but a 38" insert is probably too small for a 48" hood.

Weissman is right on - a liner is exactly that - its a space filler whereas the insert is the functional range hood. Liners are used for out of the box inserts that do not offer variable sizes. Take a look at companies such as Modern Aire who make the insert to fit wall to wall on your canopy thereby eliminating the need for the liner component.

Here is a link that might be useful: Range Hood insert and liner

Definitely ignore your salesman, who is trying to sell you the wrong item because it's all he has to offer.

You definitely want a custom sized/built hood liner, baffled, and using the best possible ventilation options you can accommodate. You definitely want a hood that is at least 48'' wide, and wider would be even better. A 38'' liner would be a joke.

That would also preferably include 10'' ducting and a remote blower with high capacity. Can't speak to jackals' brand personally but can speak to Modern Aire, who built our custom liner, which continues to delight me after three years of every day use.

Modern Aire hood liners

I believe Bluestar wants you to have 3 inches of over hang for that range. I'm not sure how that works with a hood liner, but in a regular hood that would be 54" x 27" for a wall mount and 54" x 30" in an island setup.