A specifically labelled compound is referred to as mixed labelled when the isotopically substituted compound has more than one kind of modified atom e.g. $\ce{CH3–CH2–[^{18}O][^{2}H]}$.
Sources: Blue Book, p. 515 [Terms] [Book] PAC, 1981, 53, 1887. (Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds) on page 1893 [Terms] [Paper]