A Letter of Recommendation should include information relative to the candidate’s application and attest to the candidate’s exceptional qualities and characteristics. For instance, if you’re writing a recommendation letter for someone’s job application and you worked on a project with them, you could highlight their project management ability. For an academic Letter of Recommendation, perhaps you’d want to showcase their ability to do useful and informative research. If the candidate needs the reference for a specific purpose (such as part of a rental application or to include in court testimony) then the content should remain relevant to that purpose.