كما في تشبيه ثوب بآخر في السواد، أو مقدارها
As when cloth is compared with cloth for blackness, or to show the measure of that blackness.
Condition: “This cloth is black like that cloth.” The listener learns the type of colour possessed by the first cloth.
Degree: “Its blackness is as deep as a raven's plumage.” The listener knows the quality is blackness but needs its intensity explained.
The contrast is not between a simile and a non-simile. Both are comparisons. The controlling issue is what the listener does not yet know—the nature of the quality or its amount and force. One expression may fulfil either purpose in different contexts.