What are the "allowable costs" a prevailing party can recover from an expert who has not been appointed by the court?

Among the "allowable costs" a prevailing party may recover are "[f]ees of expert witnesses ordered by the court." (Code Civ. Proc., 1033.5, subd. (a)(8), italics added.) The pertinent statute expressly provides "[f]ees of experts not ordered by the court" are not "allowable costs," except when expressly authorized by statute. (Id., subd. (b)(1); see Olson v. Automobile Club of Southern California (2008) 42 Cal.4th 1142, 1149 ["Section 1033.5, subdivision (b)(1) provides that, unless expressly authorized by law, the fees of experts not appointed by the court are not allowable as costs."].)