From Confidence level, select the level of confidence for the confidence interval. Usually, a confidence level of 95% works well.
A bootstrapping distribution approximates the sampling distribution of the difference. Therefore, the middle 95% of values from the bootstrapping distribution provide a 95% confidence interval for the difference.
For a given set of data, a lower confidence level produces a narrower confidence interval, and a higher confidence level produces a wider confidence interval. The width of the interval also tends to decrease with larger sample sizes.