"Tolerance" is the fourth component of a quality gate. Tolerance refers to an acceptable level of quality. The acceptable level could be qualitative (e.g. Yes/No) or quantitative (e.g. 97%).
Tolerance levels or thresholds are generally set by expectations of what should be observed at that point in the process for a given quality measure. The thresholds provide a range of what is acceptable quality and may have different levels of acceptance.
A powerful aspect of this component is that expectations are set in advance of the process occurring, that is they are predetermined. This prevents an erosion of quality over time because potential minor problems or changes in quality will be identified which may not have been if expectations about the level of quality had not been considered prior to the decision point.