There is an excellent introduction to Exception Sites in this article in “Planning Insight” of April 2021: https://www.planninginsight.co.uk/blogs/first-homes-and-exception-sites/.
The concept of an Exception Site derives from paragraph 71 of the National Planning Policy Framework https://www.gov.uk/government/publications/national-planning-policy-framework–2.
Rural Exception Sites are well covered in this article. https://www.communityledhomes.org.uk/planning/44
There are currently three types of Exception Site: Rural, Entry-level, and First Homes.
The Planning Insight article [see above] states that the Government intends to replace Entry-level Exception Sites with First Homes Exception Sites. Details are awaited.