Channel Codes: The Channel Code is a 3-character code that specifies the type, bandwidth of the recorded signal, and orientation of the sensor. For instance, a channel code of BHZ is used to indicate the signal came from a broadband (B), high gain seismometer (H) and is recording ground motion in the vertical (Z) direction.
A complete summary of channel codes can be found in Appendix A of the FDSN SEED Manual