From EAC-CPF tag library: "localDescription provides a means to extend the list of description elements defined by ISAAR (CPF) and specified in the EAC-CPF schema. It should be used to record structured index terms rather than discursive text. It contains a child element <term>, which may be drawn from controlled vocabularies or may be natural language terms." - http://www3.iath.virginia.edu/eac/cpf/tagLibrary/cpfTagLibrary.html#d1e4364
Out of scope for ArchivesSpace <= 1.0.