Gutenprint Code Question

Hi all,

As I go through the Gutenprint code and documentation (both developer
and user) to better understand how it works, I've got questions.

One that I am most curious about, but is not terribly important, and
which I'm convinced I can't get answered any other way than by asking
here is:

What does 'STP' mean in Gutenprint? It is the prefix for a vast number
of variables, types, etc in the code (either in all caps or all
lowercase). I'm pretty sure it doesn't mean "standard temperature and
pressure", nor "spanning tree protocol", nor "Science, Technology,
Performance (as in STP motor fuel additives)".

Thanks,

Erik