> I span space,
> but only for a stretch.
A stretch of time is a term. But as this is a spatial term and one that has its bounds. It spans space.
> I am tied,
> but without constraint.
The terms are also called bounds. But they are not bound or tied and so are not constrained.
> I live in darkness,
> but with decorum.
The luminaries are not used with the terms.
A term is a minor dignity and so it has decorum.
> I know my limits.
Again another name for boundry.
> Name me.
A name is a term.
Altair Astrology 
“The luminaries are not used with the terms.”
Ah, but Vettius Valens varies the term rulers with regard to sect; he uses different rulers for diurnal and for nocturnal nativities. In his system, though the luminaries are not used *with* the terms, they are used to determine which terms to apply.
The Egyptian terms used in conjunction with the Dorothean triplicity rulers comprise an algorithm which determines the almuten of any position in the zodiac: with this algorithm, the almuten will be either the domicile or the exaltation ruler of that position. In other words, if you have Capricorn on the ascendant, the Egyptian terms and the Dorothean triplicities will let you determine whether Saturn or Mars predominates. Morinus dispensed with this entirely and simply examined both the domicile and exaltation ruler of any point in the zodiac.
Unfortunately, I’m not quick enough to unravel your riddle!
Thanks for this insight Andrew. It puzzled me as to why the luminaries were excluded from dignity in the terms and yet had dignity by face.
Best Fishes
CarO