Standard form writes any number as a × 10k, where a is at least 1 but less than 10 and k is an integer. Large numbers have a positive k and small numbers have a negative k. To multiply or divide, work with the a parts and the powers of 10 separately, then adjust so that a is back between 1 and 10.
A rounded value hides a range of possible true values. The lower and upper bounds are half a rounding unit below and above the stated value.
Percentage error compares an approximate value with the exact one. It is always measured against the exact value and is never negative.