From the [tag guidelines](/pages/tags):
Suggestive: "Prominent low-detail sheaths or anuses (outline or hole-free shape only, like a star or circle; non-shading contrast can count as forming lines)"
Questionable: "Moderately-detailed genital shapes, sheaths, or anuses (hole-free shape with an outline, like a star/dot/line in a bump; non-shading contrast can count as forming lines)"
For the most part, actual practice is pretty similar, but here's some examples of where things stand currently:
>>537109t >>312193t >>284355t
>>1175140t >>2668543t >>1423635t
>>2621564t >>1107495t >>2671431t
Essentially, some sheaths are allowed in Suggestive when the inner detail is just a tiny dot, and "prominence" rarely kicks in (that ShiningDance image is one of only a handful affected by it).
Also note that this actually an active area of review, as about a week ago, I started discovering images missing the "sheath" tag, and from there found ratings oddities that I brought up with other staff.