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They describe the size of the actual lead or steel shot (pellets) in the shell. T is the smallest, TT is larger, and TTT (also referred to as F, for whatever reason) is the largest of the three, though these sizes are just a small part of the whole array of shot sizes available. More exhaustive explanation with other sizes included here: https://blog.thegunsandgearstore.com/shot-sizes-in-shotgun-shells/

David, it is just shot pellet size Sir. Size Chart here : http://www.rivermenrodandgunclub.com/shot-size-chart--recommendations.html