For women, sizes shown on the swing tag almost always refer to chest size in inches although some manufacturers are changing their swimwear sizing for women to introduce UK dress sizes.
The size chart below will help you work out which UK dress size corresponds to which chest measurement. As a general rule mens, boys and girls swimwear is bought by waist size. That will be the size marked on most swing tags in inches. The charts below will help you ascertain if that particular size will fit other body measurements or age.
Some mens and boys swimwear will show XS-XXXL on the swing tag. Again you can look at the charts below to see the corresponding body measurements or age. Baby swimwear is generally sold by age.