Will Robertson writes:

 > > I imagine that we will end up providing one or more very general
 > > templates with some restricted ones to solve at least parts of the
 > > issue.
 >
 > [...]
 > 
 > Since templates don't cascade (i.e., changes to higher templates don't  
 > flow on to templates and instances down the line) the idea of a  
 > restricted template seems more limited to cases when you literally  
 > need to restrict setting certain parameters in order to keep the  
 > template (or, rather, the design that the template produces) working.

they cascade as long as you don't freeze a design by instanciating it, ie if
you do the template restriction prior to do \DeclareInstance then changes will
appear.

 However, the use case for the "restricted templates" was something
quite different, so I'm not surprised that it doesn't quite fit the bill here.

The idea behind them is the following: with certain publishers you do find
something like "house style" where certain parts of the document layout is
predefined, while other design elements are flexible and allow variation
depending on the target document class. For example, the publisher might want
to always use certain fonts, etc.

Now rather than asking them to take basic templates, copy them  and changing
their implementation to take out some of the basic customization
possibilities, the idea was to offer a mechanism through which come aspects of
templates could be frozen. 

The house style, would then provide a set of restricted template and document
classes written in the house style would produce their design by restricting
themselves to use only templates from the house style.
The way things are written  the unrestricted templates would still be
available for those people who want to use them, so this is more a guiding
principle and not a real prevention mechanism.

The other use case is to set some common values for instances by taking a
template restricting it in the design and then reuse the restricted
template several times for instantiation, e.g.,

\DeclareRestrictedTemplate{sectioning}{xxx}{vertical-heading}
      { font = \fontfamily{ptm}\fontseries{b}\selectfont ....}

\DeclareInstance{sectioning}{chapter}{xxx}{...}        % no need to set font 
\DeclareInstance{sectioning}{section}{xxx}{...}        % any more 
\DeclareInstance{sectioning}{subsection}{xxx}{...}


Both use cases are really for the design layer and aren't really applicable
for user overwrites on individual documents.

Whether or not that is really a good idea or only snytactic sugar not worth
having, I'm not so sure these days, but that was the original motivation.

frank