The primitive data types prefixed with "u" are unsigned versions with the same bit sizes. Effectively, this means they cannot store negative numbers, but on the other hand they can store positive numbers twice as large as their signed counterparts. The signed counterparts do not have "u" prefixed.
research : comparision between int and uint
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reference
https://stackoverflow.com/questions/3724242/what-is-the-difference-between-int-and-uint-long-and-ulong