What is the test for discretion of the Court of Appeal in deciding whether to grant relief under section 473 of the Code of Civil Procedure?

It is a familiar principle, grounded in the nature of the respective functions of trial and appellate courts, that a trial court has broad discretion in acting upon requests for relief falling within the scope of Code of Civil Procedure section 473 and will not have its exercise of such discretion set aside except where it has manifestly been abused. At the same time " 'The discretion intended, is not a capricious or arbitrary discretion, but an impartial discretion, guided and controlled in its exercise by fixed legal principles. It is not a mental discretion, to be exercised ex gratia, but a legal discretion, to be exercised in conformity with the spirit of the law and in a manner to subserve and not to impede or defeat the ends of substantial justice ' " (Benjamin v. Dalmo Mfg. Co. (1948) 31 Cal.2d 523, 526, 190 P.2d 593.)