ComputerNumber[x]

gives the ComputerNumber object equivalent to the ordinary number x in the arithmetic currently in effect.

ComputerNumber[sign,mantissa,exp]

gives the ComputerNumber object whose value is sign mantissa b^exp, where b is the base in the arithmetic currently in effect.

ComputerNumber[sign,mantissa,exp,value,x]

is the complete data object that makes up a computer number.