It says for a generous fit, buy one size up. It should say, to be able to breath, buy one size up, to be able to move, buy two sizes up. I bought one size up from my normal size and couldn’t button it. Partly because they barely touched and partly because if I had reached to my center I would have ripped the shoulders out.
I bought these when offered earlier. Good material and fairly well made. There is no pocket. I’m a bit of a gym rat so i bought a size up. Snug across the chest, tight arms, and “tailored” at abdomen. Wife bought me 2 sizes up and shirt was comfortable but L O N G. If you are of average or slender build, these are nice shirts. Well worth it. I gave my first batch to my nephew and he got a lot of use from them. If you have any build or carry winter weight i would caution.