Class is defined by relation to property, wage labor, and the mode of production. The proletariat are those that are almost or entirely propertyless, obliged to sell their labor for wages. Petty bourgeious often is associated with small business owners, but can include those that do wage labor but don't have a substantial immediate concern if they were to lose their job, the middle class is the modern equivalent basically. It's not a hard and fast thing and there is a general trend, outside of great moves like the shift towards stocks and pensions tied to mutual indexes and finance capital, that results in the proletarization of the petty bourgeois. Other classes have been eroded in time-peasantry no longer exist in a great deal in most countries, lumpenproletariat still exists as well.

PMC can refer to a general group-it's issue though is the tying together of managerial positions and proletarian positions like nurses, whose relation to production is considerably different, due to cultural signifiers. Some people in the Chapo sphere like Matt are aware of this, but take the position that since America is so devoid of material politics, that it's a useful lens to analyze how partisan politics works in the US, and attempts to break from electoral politics like the nascent left and the tensions within it.