C-Class pages have substantial content and will often be useful to readers, but they may have significant prose and style problems, may require cleanup, may not cover all significant areas of the topic, or may not explain the concept in a way the target audience will reliably understand.
Contrast:
Start-Class pages, which are sufficiently unfinished that they should be viewed by editors rather than readers.
B-Class pages, which are reasonably comprehensive, have no major stylistic issues, and explain the concept in a way which seems likely to be often enlightening to the target audience.