There are different versions of certain aircraft such as the 737-200/300/400/500/600/700/800/900. The second number shows which version it is, for example: the 737-900 has the longest fuselage of the 737's and the 737-500 is the smallest. In some cases, letters are used with the numbers: 767-300ER, or without the numbers: 747SP or C-130J.
Hope this helps.
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