No. It's based on historical US stock/bond market returns over rolling 30-year periods and had roughly a 95% historical success rate in the original study. Future returns, inflation, sequence-of-returns risk, and your own spending flexibility can all change the outcome.
A lower rate (e.g. 3.5%) is more conservative, useful for very long retirements (30+ years) or uncertain markets. A higher rate (e.g. 5%) assumes a shorter retirement or more flexible spending.
No. This calculator only figures out how much your investment portfolio needs to cover your expenses. Subtract any other guaranteed income (pension, social security) from your expected expenses first for a more accurate number.