I'm guessing that you've looked at this in Wikipedia and seen "Displacement: 4,900 t (4,800 long tons; 5,400 short tons)". There are three types of measurement being used: metric, imperial and US customary. In this example 4,900t is a BIPM accepted non-SI term for 4.9 Gg or 4,900 Mg.
In the UK the stone weights 14 lb and there are 8 stone to the hundredweight of 112 lb. The US has never used stones, and uses 100lb to the hundredweight. To distinguish the two the imperial 112 lb is called a long hundredweight whereas the US 100 lb one is a short cwt. In both systems there are 20cwt in a ton, but of course since the hundredweights are different the tons are as well. A long ton (UK) is 2,240 lb, a short ton (US) is 2,000 lb.