It is useful for extracting specific level from the data with vertical levels.
cdo sellevel,2 in.nc out.nc
then, you can extract the data with only 2nd vertical level.
however, sometimes exact vertical level is required for NCEP/NCAR data sets.
Like following statement,
cdo sellevel,500 in.nc out.nc
then, you can get the data of 500 hPa.
cdo sellevel,25/900 -selname,vwnd in.nc out.nc
So, Simple 🙂