Channel naming follows the SEED standard as outlined in Appendix A of the SEED Manual:
The first letter is the band letter and implies a sample rate. For short-period seismic instruments with natural period < 10s (short period), the letter S is used for sample rates between 10-80sps, with E used for sample rates from 80 and below 250 sps. For broadband instruments (natural period >=10s) the band letters B and H are used for those sample rates. There are more letters for higher and lower sample rates.
The second letter is instrument type, where the common letters are H for high-gain instrument, L for low-gain and N for accelerometers. There are many others, for example D for pressure (barometers) or depth gauges, K for temperature, etc.
The third letter of the channel is direction where naming is usually Z/N/E components, though some networks are using orthogonal components Z/1/2 where the orientation is specified in the metadata.