So I have seen power supplies that produce e.g. -48V DC (used e.g. in telecom but probably not only). For sure, that power supply is floating and in no way is negative terminal connected to earth. So, what doesn't make sense to me is why we need to specify minus sign on power supply? What does it actually means?
Current flows from positive to negative (or actually from higher potential to lower). So, if we have negatiive power supply - ok, current will flow from 0V to -48V, so it would be reversed. But then, if you have +48V PS - you swap terminals and got -48V. What's the actual difference? Is it sink vs source of current, or is it related with human safety? For me from engineering point of view it somewhat doesn't make sense unless you need both positive and negative voltage rails.
I do get this notion with ATX power supply, where we have +12V rail and -12V though
Upd 1 found this link which indcate why telecom uses negative voltage but that doesn't explain what it really is. https://deltaelectronicsindia.com/faq-items/why-does-telecom-use-negative-voltage/
Update 2 Another thing came to my mind as result of thought experiment... So we have polarized DC MCBs. Those should be connected correctly in terms of polarity, otherwise those can catch fire because arc won't be properly extinguished. So, how do you properly connect those? Something tells me that 0V should be connected to "+" terminal and -48V for "-"