EDIT

Andreas Kohlbecker wrote:

this behavior should be activated when TaxonNamePopupEditorMode.NOMENCLATURALREFERENCE_SECTION_EDITING_ONLY is set in the name editor.

this was a wrong guess!

The solution to this rather is related to correcting the field visibility with is defined in the eu.etaxonomy.cdm.model.reference.ReferencePropertyDefinitions

for section the visibility is defined as:

iSection = merge(iReference, iWithAuthorAndDate, iWithDoi, iNomenclaturalReference);

this makes not much sense as sections are always part of a bigger enclosing published unit which has DOI, date and so on.

This brings us back to the old discussion whether we need a different type of section which is dedicated to sections like paragraphs in journal articles and books.

Currently ISection is defined correspondingly to the TDWG ontology PublicationTypeTerm "SubReference".
I thing this is not correct as there is also a "section" term in the TDWG ontology:

<owl:DatatypeProperty rdf:ID="section">
      <rdfs:label>Section</rdfs:label>
      <rdfs:comment>Some publications are split into sections. This can be signified here.</rdfs:comment>
      <rdfs:domain rdf:resource="#PublicationCitation"/>
      <rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#string"/>
      <rdfs:isDefinedBy rdf:resource="http://rs.tdwg.org/ontology/voc/PublicationCitation"/>
   </owl:DatatypeProperty>

I think we actually need both of them "SubReference" and "Section".