Definition: Confidence that the containing instance actually occurs or exists in vivo, usually a statistical measure. The xref must contain at least on publication that describes the method used to determine the confidence. There is currently no standard way of describing confidence values, so any string is valid for the confidence value. In the future, a controlled vocabulary of accepted confidence values could become available, in which case it will likely be adopted for use here to describe the value. Examples: The statistical significance of a result, e.g. "p<0.05".
Source:http://www.biopax.org/release/biopax-level2.owl#confidence