15N,13C-labelling is commonly referred to as double labelling. The protein is produced by expression from bacteria which are grown on minimal medium supplemented with 15NH4Cl and 13C-glucose.
This is a very common form of labelling and enables straight forward assignment of both the backbone and side-chain 1H, 13C and 15N atoms using triple-resonance spectra. A high proportion of these assignments are required if you wish to calculate the structure of your protein accurately.
L.P. McIntosh and F.W. Dahlquist (1990) Quart. Rev. Biophys. 23 1-38. (Link to Article)