Greater social context would be useful. It’s hard to accurately imagine lifestyles denoted simply by “middle” or “upper-middle” class. They are effectively meaningless terms.
All self-described "middle class" status tells us is they're not homeless, and also capable of easily recalling how many homes they own.
So everyone uses 'middle class' unless they own a jet or an island. It's the safe choice.
It means 'derives their income from capital', just as the working class 'derives their income from labour'.
The middle class walk both lines, seeing some of their income come from capital, and some from labour.
Realistically, most people talking about class probably are middle class.
Thus you end up with four categories: Those that cannot meet their needs, the poor, those that meet their needs exclusively through wages with no significant capital, the lower-middle class, those that need to work to make ends meet but can offset some of that through the leverage of some amount of capital, a spectrum of the middle to upper-middle class, until you reach the upper class/rich/bourgeois, where their needs can be met exclusively through the leveraging of capital.
The guy who works a 9-5 but invests some of their paycheck in publicly traded stocks has a lot more class interest in common with all the other wage workers than they do with a small business owner.
Sure, and they'd still be proletarians, even if, in many cases -- and this will often be characterized as "middle class" or even "upper middle class" depending largely on income in American discussions -- part of the proletarian intelligentsia. There's also a wider array, now -- though its still a small share of the population -- of patterns of economic participation that involve loosely balanced (in the sense that both are important, if not equal, contributors) labor (either wage-labor for another or labor applied to your own capital in one way or another) and capital dependence, and which are still petit bourgeois, and which is still the distinct middle class between the proletarian and the haut bourgeois, not the false middle-income workers "middle class" invented to divide the class dependent on wage-labor.