magnetic flux density at the nucleus (from experiment) in those cases in which the magnetic hyperfine interaction can be described by an effective field. In other cases the vector components of the magnetic hyperfine interaction should be reported if possible.
Source:
PAC, 1976, 45, 211. 'Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data' on page 214 (https://doi.org/10.1351/pac197645030211)