What is the test for objecting to questions which only go only to credibility of the witness?

The respondent in objecting to the questions submits that questions which go only to the credibility of a witness should not be permitted in an examination and relies on the case of Shickele v. Rousseau (1965), 1965 CanLII 742 (BC SC), 53 W.W.R. 567 (B.C.S.C). With respect, that case is distinguishable in that the defendant there was a medical doctor and was asked certain questions on discovery which were objected to on the grounds that they required him to state his opinion. The defendant answered questions as to what treatment he gave to the plaintiff but refused to answer questions as to what was proper treatment for a case of that type. The case went on to hold that when a witness who is not himself a party to an action but appears representing a party, questions should not be put to him to elicit answers based on some special skill or knowledge that he may possess as an expert and the party he represents should not be bound by any admissions based on such special skill and knowledge. But since the defendant himself was an expert, the questions were proper under the circumstances. The case went on to state that questions which go only to credibility of the witness are not permitted, but apart from that the scope of an examination for discovery is wider than that which is permissible at trial and is limited only by the requirement that questioning must be relevant to material issues.