A B-Class page is fairly comprehensive, with good prose and structure; but needs further work to reach the A-Class requirement for engaging, professional, intuitive explanations, checked by asking for detailed feedback from both the target audience and reviewers.
B-Class pages are:
Comprehensive, covering all major areas of the topic without significant omissions or inaccuracies.
Fairly well-written and in line with the style guidelines.
Reasonably well-organized, with similar concepts grouped together and reasonable section ordering.
Aimed at an audience, with a level of technicality, jargon, and notation appropriate for them.
Good for people wishing to learn the topic (well-explained, using examples, links, and visual aids as appropriate).
Well-summarized, in ways appropriate for the most likely audiences.