Solve the Subtraction Equation: Finding the Missing Number in 6-?=6

To solve the equation $6 - ? = 6$, we begin by understanding that we need a number which, when subtracted from $6$, produces no change.

Recall the property that any number minus zero results in the original number:

$a - 0 = a$

In this problem, if $a = 6$, then:

$6 - 0 = 6$

Therefore, the missing number must be $0$ to maintain equality.

Checking the provided options, the correct answer is choice $'0'$.

Thus, the solution to the problem is $\boxed{0}$.