Each product has it's own custom sizing chart. Some manufacturers use the garment's sizing while others use body sizing. If a chart is displaying one number per box, then the sizing chart is using the actual size of the garment. For example, half chest for an adult small is 18 inches. This means the adult small item meansures 18 inches from one armpit to the other.
If a chart is displaying a number range per box, then the sizing chart is using body sizing. For example, chest for an adult small is 35-38 inches. This means, if your chest measures between these numbers, then the item should fit you.
All manufacturers use thier own sizing guides when creating the garments. Please consult every garmet's sizing chart as they will not all fit the same.