The semi-sextile joins signs that stand next to each other, and adjacency is exactly its difficulty. Neighbouring signs share no element, no modality and no polarity, and in the sign-based scheme that governs traditional practice they are in aversion, unable to perceive one another. On that account thirty degrees is not a small aspect but no aspect, the same verdict the tradition passes on the quincunx and for the same reason.
It entered the working repertoire through the harmonic route rather than the classical one. Thirty degrees is a twelfth of the circle, so the semi-sextile is the defining division of the twelfth harmonic, and once aspects were understood as harmonic divisions rather than as relations between signs there was no principled reason to exclude it. That shift is the whole story of the minor aspects, and the semi-sextile is its clearest case: an angle with no ancient standing that is now printed on most chart wheels.
Two practical consequences follow. The orb allowed is very small, commonly one or two degrees, because a wider one would blur it into the conjunction on one side and into nothing on the other. And the reading given to it is correspondingly modest: a slight friction between adjacent and unlike things, a sense of one condition giving way to the next. Writers who take it seriously describe it as the aspect of a threshold. Writers working from traditional principles do not describe it at all, and a reader should know which kind of source is in front of them before weighing what it says.