PLY ratings can be quite confusing as they don't necessarily mean what they used to! A 'PLY' rating used to state how many plies (layers) of rubber were woven into the construction of the tyre. The more plies of rubber the tyre had, usually meant the tyre was capable of carrying more weight.
However, modern tyre manufacture has introduced a higher level of plastics and polymers such as nylon and polyester to increase the strength, durability and weight carrying capacity of the tyre. By using these modern materials tyres can out perform the old PLY rating indicator with many less layers making the old scale of measure a little misleading.
Modern PLY ratings tend to indicate the strength and carrying capacity of the tyre as if it had the same number of plies of rubber used from the older scale. Usually, tyres with a higher PLY rating can carry more load, but do cost more as the rating increases.
PLY ratings should be correct for the machine they are being used on. It is good practise to have an equal or higher PLY rating than the manufacturers recommend, but although the durability and carrying capacity often increases with PLY rating, the comfort of the tyre is often reduced due to the additional stiffness of the tyre.