The naming of atoms for the NMR Exchange Format (NEF) mostly follows the IUPAC convention.
The images below show all the possible NEF atom names for the atoms in the 20 natural amino acids. The x/y notation is for situations when the assignment is non-stereospecific and the % notation for situations where chemical shifts are degenerate. Note that for amino acids such as Val or Leu the methyl protons will usually be degenerate within a methyl group (e.g. HG1% and HG2% for Val), but that both methyl groups may also be degenerate and have the same chemical shift (in which case all Val methyl protons become HG%). Please also be aware that the x/y notation is arbitrary, i.e. HBx does not always correspond to HB2 and HBx to HB3 etc. This correspondence is simply used in the graphics for convenience and consistency. A pdf file with all atoms names is available for download.