what does capital U stand for in C


In the following c code, what does U stand for? I get that the code moves left shifts 1 by x bits but why is the one necessary?

/* Bitwise operation helpers */
#ifndef BIT
#define BIT(x) (1U << (x))


Integer constants are treated as int by default. By adding U it will be treated as unsigned int instead. See the "U & L formatters" section of Integer Constants - Arduino Reference for more information.

Ok thanks- why does it need a bitwise operation require an unsigned int? Sorry if that is an obvious question but I don't get it. Thanks