\set vs \override

[David Kastrup]

There is a chapter "set vs override" in the manual.

I am afraid that I fail to grasp the difference from the chapter.

It says: "There are actually two different kinds of properties."

But then it says

    Context properties can change value over time while interpreting a
    piece of music; `measurePosition' is an obvious example of this.
    Context properties are modified with `\set'.

So far so good.

    There is a special type of context property: the element
    description.

If the element description is a _special_ type of context property, that
would imply that there is just _one_ different kind of properties.  It
completely fails to mention _what_ makes the element descriptions
special and not settable with \set.

    These properties are named in `StudlyCaps' (starting with capital
    letters).  They contain the `default settings' for said graphical
    object as an association list.  See `scm/define-grobs.scm' to see
    what kind of settings there are.  Element descriptions may be
    modified with `\override'.

So why can't we use \set with element descriptions?  And where is the
difference to context properties changing values?  After all, the
principal purpose of \override is also to change the value, and
\once\override very obviously changes behavior over time while
interpreting music.

Maybe I am dense.  Can somebody clarify?

-- 
David Kastrup