[Model] section 3.2.5

Sequentially going over the draft ...

and as before, if theres some consensus i will put this in the tracker.

Summary: this is also a readability issue:

Section 3.2.5 references Elements in order to talk about their
properties. 
Since "Elements" are such a central concept in LFB modelling, I think it
would make a lot of sense to have a short subsection on LFB Elements
(derived from the first paragraph) with proper reference for more
details (currently described as "elsewhere") followed by a subsection on
Element properties.

cheers,
jamal