Descriptions of behaviours consist of examples, or incidents, which are relevant to a given qualification. Such descriptions relate to observable events and are usually verifiable by others. Achievements refer to degrees, awards, professional accreditations, publications, etc. Activities refer to memberships, participation in various groups, etc. Opinions are distinguished from the other two types of information in that they involve a value judgement, an evaluation of some sort, based on facts.
Opinions go beyond the facts.
Different considerations are introduced with each type of information. Descriptions of behaviours allow you to judge for yourself the relevance and importance of the information for the qualification being assessed. Descriptions of behaviours are often useful to support evaluations. The most important concern with respect to descriptions of behaviour is the representativeness of the observations. You will want to know not only what the candidate has been capable of, but what he or she typically did over a period of time.