The working class is people who sell their labor for a wage. The working class used to have a much more obvious relationship to the means of production, but nowadays (especially in the US) the imperial core has transitioned largely into a service economy, while the means of production have largely been relocated to the imperial periphery where labor can be expoited more easily. None the less, the working class remains people who sell their labor for a living, whether it is in a manufacturing, agricultural, or service context.

The PMC is the "Professional Managerial Class." Under Marxist theory, the contradiction between the Proletariat (the working class) and the Bourgeoisie (the capitalist class) forms the primary contradiction, but there are obviously many more contradictions which exist within society, and many more subclasses between which these contradictions exist. For instance, there exists a "labor aristocracy." Definitionally, the labor aristocracy are proletarians, but they are proletarians situated in such a way within the global system of capitalist imperialism where they reap many of the benefits of imperial hyper-exploitation of the proletariat of the Global South. This is an example of a secondary contradiction within subclasses of the Proletariat.

Likewise, there are contradictions within the Bourgeoisie. Most commonly, the Bourgeoise is divided into two prominent subclasses: the Haute Bourgeoisie, and the Petite Bourgeoise (the big and small Bourgeoisie). You could think of the Haute Bourgeoise as the "captains of industry." The Amazons, Facebooks, and Foxconns of the world. The Petit Bourgeoise on the other hand are the small business owners. They still accumulate capital by paying workers less than the value produced by their labor, but they are basically rowboats floating in an uneasy sea dominated by battleships.

The PMC is a social phenomenon which occurred prominently in the United States along with the phenomenon of suburbanization. At one point we had industrial cities with factories surrounded by proletarian slums. Class divisions were very pronounced geographically. You'd have a factory, then everyone who worked at that factory would live around the factory, and from these communities there would grow proletarian institutions like union halls.

Then the automobile and white flight happened. A lot of these urban factories shut down and moved to the newly built suburbs. A lot of bourgeois and proletarians settled in the suburbs simultaneously, and the automobile allowed workers to live much farther away from the means of production, much farther away from their coworkers and union comrades. Before you knew it, labor aristocrat proletarians were living across the street from petite bourgeois buisiness owners - while being atomized from their co-workers. This atomization dissolved classic proletarian institutions like centrally located union halls.

Time, and a lot of cold war propaganda passed. People came to recognize their status based on their income levels, rather than their relations to the means of production. You could have a proletarian X-Ray technician living on one side of the street making 120k/year and a petit bourgeois proprietor of a landscaping company living across the street making 100k/year. Next door you could have a middle-level manager from a Haute Bourgeois regional corporation making 80k/year. These people grew to identify with each other and their suburban lifestyles rather than their class. This is the origin of the PMC. The nexus of the proletarian labor aristocracy, the Petite Bourgeoise, and the humble servants of the Haute Bourgeoisie living in colocation, isolated from their class cohorts, and identifying with a shared lived existence revolving around homeownership, lawns, automobiles, grilling with the neighbors.

The existence of a "Professional Managerial Class" is a relatively new and somewhat disputed development in Marxist theory. The term was only introduced in the 1970s by John and Barbara Ehrenreich. I have only read summaries of their work, but based on those summaries I find their perspective to be a bit crude. Personally, I don't consider the PMC to be a new, distinct class. Rather (as I described it), I understand it to be temporary phenomenon of class collaboration. The PMC was a product of the post-war boom. The "golden age." But with the declining rate of profit, and the expiration of the United States role as sole industrial producer in the wake of WWII, the conditions which allowed the American industrial proletariat to rise to such commanding heights are gone, and will never return.

Due to the forces of international competition and financialization, the labor aristocracy which made up the proletarian section of the PMC is becoming irreversibly proletarianized yet again. The teachers, nurses, doctors, technicians, office workers, etc are all trending on a decline. A college degree isn't the golden ticket to middle class that it used to be, and the dependable union pensions on which an honest worker could retire are drying up. The suburban homeowner lifestyle which formed the basis of the PMC has become practically unattainable for Millenials and Zoomers, and before we know it, there won't be a PMC at all.