DOCBOOK-APPS: DSSSL Variables

Hi!

What's the reason that some variables of the Modular DocBook style sheets
are enclosed in percent signs, others are enclosed in dollar signs and
still others are not surrounded by any special signs at all? Is this some
convention that is followed throughout the style sheets? If so, what's the
meaning behind all this?

Thanks in advance!

Greetings,

	Holger