Spire by Galaxy Men's Puffer Hoodie with Detachable Hood

Can somebody explain how have these measurements of chest 44" for S, 46" for M, 48" for L etc. been taken?
I have not seen standard US/UK sizes like this because usually M is 40" chest.
I wear a 42" shirt. What size of this jacket should I get, and what’s the return policy if the size I order does not fit well?

Hi there. Here’s a note from the vendor:

These measurements are your chest size. Instead, the measurements are the spec of the jacket.

If you were to measure the circumference of the chest of the jacket the measurement for size small all the way around would be 44”, those extra inches are to account for the clothes that would be worn under the jacket plus a little extra room for comfort, if the jacket was designed with a 40-42” chest measurement then it would be too snug or slim fit, this jacket is a modern fit.

With regard to a t-shirt, the measurements are only a couple of inches bigger than your chest size because t-shirts are more of a snug or fitted fit, as opposed to a jacket that is designed to be a bit more loose with some room for comfort.