I saw this in:

Minutes of the Subgroup "Structure of descriptive data" workshop at
TDWG 1999 in Harvard

Diederich's "Basic properties"
General agreement was reached that a direct application of the
"structure-property-value" model would be too restrictive, applicable
mainly to morphological descriptions. A more general model, including
cultural/physiological and molecular descriptions should be
developed. The term "feature" was proposed as a more general
replacement for structure.
This feature-property-value triology is what I had in mind the first time I read about Delta.
A few remarks:
  1. the feature can be a hierarchy, like:
leaf/lamina/abaxial_surface/vein_islands/indumentum/density All together this is a sound and state-of-the-art XML foundation; of course a lot of details are left.

Cheers

JMV