What does this mean "As there is no specific source rule in section 9 of the Income Tax Act, case law applies."

This means that since there is no specific guideline or provision in section 9 of the Income Tax Act regarding a particular issue, decisions and interpretations made by previous court cases will be used as the basis for resolving the matter. In other words, decisions made by the court in similar situations in the past will be considered in determining the appropriate treatment of the issue in question.