A new FAQ page devoted to the topic of private-use characters, noncharacters, and sentinels has been posted on the Unicode web site. This FAQ aims to clear up confusion about whether noncharacters are permitted in Unicode text, and how they differ from ordinary private-use characters. The recently published Corrigendum #9: Clarification About Noncharacters makes it clear that noncharacters are permitted even in interchange, and the new FAQ page addresses some of the fine points about their usage and about differences from other types of Unicode code points. The brief mentions of noncharacters in other FAQ pages have also been updated accordingly.
Are you unclear about what Unicode “noncharacters” even are? The new FAQ page
also answers basic questions about noncharacters and private-use characters, and
provides a bit of history about how they came to be part of the Unicode