https://en.m.wikipedia.org/wiki/Class:_A_Guide_Through_the_A...
His “Middle” is characterized by a lot of class-anxiety and especially the twin characteristics of wanting to appear like the upper-middle or upper classes, while having very wrong ideas about how those two classes “signal”.
His upper-middle is basically professional-class (doctors, lawyers, professors, bankers, finance) prep school sorts. Same as the main demographic covered in Birnbach’s The Official Preppy Handbook
When I was a kid I moved from a rich suburban New England town to one that was considerably more working class. When I lived in the rich town, no one talked about how much money the other kids' parents had, I guess because everybody was rich. When I lived in the working class town, everybody knew who the rich kid was and who the poor kids were, and where you shopped could get you admired or bullied. Wearing clothes from K-mart was a fast track to becoming a pariah.
Fussellian middle class in a suburban school: will call your kid poor and bully them if they don’t have a recent phone by 6th grade (often much earlier)
Fussellian upper-middle in expensive private school: may not have a phone themselves until high school (handy for keeping in touch when at college-application-building extra curriculars), won’t make fun of kids whose parents don’t give them phone.