It may be interesting to briefly compare the expectation from surface magnetic fields produced via the FeCZ to that for fields produced by other means. Surface fields produced by convective cores mentioned above, even if for massive stars the buoyant rise of magnetic fields from the convective core seems to be unlikely \citep{2004MNRAS.348..702M}. In contrast to the sub-surface FeCZ, convective cores are prevalent in all stars above about 1.2\({\,\mathrm{M}_\odot}\). It has been found that the longer lifetime of stars of lower mass may favor the drift of fields produced in the core to the surface \citep{1978AA....68...57S,2003ApJ...586..480M}. Therefore, the expected trend is opposite to that found for fields produced by the FeCZ, where surface fields may occur only for stars above a critical mass (or luminosity), and stronger fields are found for more massive stars.

On the other hand, in contrast to fields from the FeCZ, magnetic flux tubes produced in the core may carry CNO-processed material to the surface. This might thus constitute a mechanism to explaining nitrogen enrichment in slowly rotating early B stars \citep{Mba06,Mgb08,2008ApJ...676L..29H}. Strong fossil magnetic fields are thought to persist in only a fraction of massive stars and may lead to, among other phenomena, highly anomalous surface chemical compositions, wind confinement, and variable X-ray emission \citep{2006AA...451..195W,2005ApJ...630L..81T}. Those strong features can clearly not be produced by fields originating from the FeCZs.

Finally, magnetic fields produced in differentially rotating massive stars by the Spruit-Taylor dynamo \citep{Spr02} may transport angular momentum and chemical species \citep{hws05}. These fields are predominantly of toroidal geometry and would quickly decay near the stellar surface, and are thus not thought to lead to observable fields at the stellar surface (but see also \citet{2005MNRAS.356.1139M}).