= beyond

Eighteen percent gray reflects 18% of the light shining on it. They're used in densitometry as a reference point because they're in the middle of the reflectance range**.

Kodak sells 18% grey cards which have a 90% reflectance white surface on the reverse side (for use in dark light situations when a light meter does not respond to the 18% gray). Photographers simply increase the exposure by two and a half stops to compensate.

Ninety percent reflectance is fives times the reflectance of 18% (18 goes into 90 five times). A "stop", in photography, is a doubling or halving of the exposure. From 18 to 90, the first stop is times two, the second is times four (2*2), and a half-stop to make 5. In your case, they're assuming a reflectance of around 85%.

Reproduced correctly, a white card should reproduce as a detailed white. Beyond "detailed white", there is "bright white" (white with no details visible), at least a full stop beyond "detailed white", and specular white (very very bright white, like the sun). The differences beyond two-and-a-half stops above 90% become so small that, for all intents and purposes, they become the same.

Trust me, I know my photography (20 years in the business including a long stint as chief photolab technician)
**I realise it sounds weird to say that 18% is the midpoint instead of 50%, but 18% gray is the point where transmission and reflection meets in densitometry: they both read as 0.75 whether its a print, a slide, or a negative.