Across the infinite and much-varied multiverse, one of the few constants to be found between planes of existence is the existence of a thing called a soul. Souls are required for sentient life, and every sentient being has a soul. As a homeplane drifts through the multiverse, it has a “gravitational pull” which passively accumulates souls - or, more accurately, “proto-souls”1). The question of where exactly they originate is difficult to answer, but it seems that they are simply generated as part of the natural laws of the multiverse. These proto-souls may be used by the custodians of the homeplane to foster more sentient life, or kept aside for trade with beings from other universes, such as at Emporium. The more life a homeplane has, the more proto-souls it collects, leading to increased growth over time2). Additionally, once a soul has been incarnated into a living being, it becomes “marked”: forever altered by that universe’s metaphysical properties and rendered unsuitable for multiversal trade3). In this way, it is neither ethically nor economically viable to harvest the souls of the already-living for financial gain.
On the other hand, the proto-souls are more like the potential for new (sentient) life; a vital building block for Entities to bring more complex forms of life into their worlds. They are multiversally viable, and hence are valuable - either for the ability to nurture sentient life in its own right or for the investment opportunity offered by the increased accumulation rate of further proto-souls.
As previously stated, only proto-souls are viable for trade, but since that is a bit of a mouthful, everyone simply refers to them as “souls” in context4).
Emporium is a place frequented by beings immensely powerful in their homeplanes. Such Entities can often trivially obtain large numbers of souls simply by rummaging under the cushions of their metaphysical couches. Thus, anything less than a (sometimes literally) astronomical number of souls is considered pocket change. The loss or gain of such a small amount of souls will usually not significantly impact a homeplane. Items such as snacks, trinkets and souvenirs5) may (within reason) be trivially obtained for a handful of souls. See this page for more details on buying items.
On the other hand, larger and more valuable items may cost a very large number of souls indeed. For these - the items that Entities specifically come to Emporium for - a more manageable currency is required. While Emporium often trades with customers using a barter system, it is still sometimes useful to introduce an intermediary currency - for example, when paying employees or in compensating customers in recognition of services rendered.
To this end, Emporium has introduced SoulCoin, a form of an in-store credit. A single SoulCoin represents a very large quantity of souls, making the value of a single SoulCoin approximately on par with the more desirable items. A character's 'pocket change' of souls is significantly less in value than even a single SoulCoin, though it is possible to start the game with one SoulCoin by taking the relevant Quirk. Again, see this page for more details on buying items.