What does this small D mean?

[wild nacelle]

Cant find it anywhere

[weak sail]

from what I can find, that subscript seems to represent the universe in which C is taken as a complement of.
for example, $\overline{A}_B$ is the complement of A \textbf{taken relative to B} (that is, $\overline{A}_B = B \setminus A$).

(you may want to wait for a second opinion!)

[steel turret]

ʚ littledove ~ Hyposelenia ɞ

[frigid canopy]

rather unusual notation for the relative complement of a set if it is one. though given the context I think that may be the only interpretation.