Final answer:
In binary addition, '1' is significant and carried over to the left when the sum at a given digit exceeds the binary base of 2, turning the exceeded position into a '0' and contributing a '1' to the next significant position.
Step-by-step explanation:
In binary addition, the digit '1' is significant and carries over to the next significant digit to the left when it exceeds the base of 2, similar to how in decimal addition '8' rounds the sum up if the next digit is a '1', resulting in the '1' being dropped. For instance, when adding 1 + 1 in binary, the sum would be '10', where '1' is carried over and the digit '0' remains. This is equivalent to adding 10 to a decimal number and then dropping the unit digit, similar to rounding up a number.
If this binary addition results in a sum equal to or greater than the base of 2, the rightmost digit becomes '0', and '1' is carried over to the next significant digit to the left. This process continues for each digit involved in the addition if there's a carryover.