All 4 Uses

subtle
in
Flags of Our Fathers
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"It was a subtle destruction of your civilian mentality," is how Robert Leader remembered it.†
Chpt 3.
The intuitive brilliance he'd shown in childhood now focused acutely on the inner subtleties and structures of Marine life.†
Chpt 4. *
Throughout the six months at Pendleton, all of Spearhead's boys, veterans and new Marines alike, sensed a subtle but unmistakable new climate of respect from the brass.†
Chpt 5.
This ultimate public investiture had transported the three across a subtle boundary.†
Chpt 18.

Definitions:

(1) (subtle as in: a subtle poison)
working in an indirect or hidden way
(2) (subtle as in: a subtle shade of blue)
understated so as not to draw excess attention
(3) (subtle as in: a subtle difference or thinker)
not obvious, but noticeable with enough insight and knowledge

or:

able to notice or understand things that require insight and sensitivity
(4) (meaning too rare to warrant focus)