The C-y command yanks the most recently killed text back into the Calculator. It pushes this value onto the top of the stack regardless of the cursor position. In general it re-parses the killed text as a number or formula (or a list of these separated by commas or newlines). However if the thing being yanked is something that was just killed from the Calculator itself, its full internal structure is yanked. For example, if you have set the floating-point display mode to show only four significant digits, then killing and re-yanking 3.14159 (which displays as 3.142) will yank the full 3.14159, even though yanking it into any other buffer would yank the number in its displayed form, 3.142. (Since the default display modes show all objects to their full precision, this feature normally makes no difference.)
The C-y command can be given a prefix, which will interpret the text being yanked with a different radix. If the text being yanked can be interpreted as a binary, octal, hexadecimal, or decimal number, then a prefix of 2, 8, 6 or 0 will have Calc interpret the yanked text as a number in the appropriate base. For example, if ‘111’ has just been killed and is yanked into Calc with a command of C-2 C-y, then the number ‘7’ will be put on the stack. If you use the plain prefix C-u, then you will be prompted for a base to use, which can be any integer from 2 to 36. If Calc doesn't allow the text being yanked to be read in a different base (such as if the text is an algebraic expression), then the prefix will have no effect.