selectively labelled 

https://doi.org/10.1351/goldbook.S05556
An isotopically labelled compound is designated as selectively labelled when a mixture of isotopically substituted compounds is formally added to the analogous isotopically unmodified compound in such a way that the position(s) but not necessarily the number of each labelling nuclide is defined. A selectively labelled compound may be considered as a mixture of specifically labelled compounds.
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]