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.
CH3–CH2–[18O][2H].
Sources:
Blue Book, p. 515 (https://www.acdlabs.com/iupac/nomenclature/)
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1893 (https://doi.org/10.1351/pac198153101887)