Currency

There is 2 types of Currency in the game:

Sx
You use Sx to buy things (mostly). Sx are crystals. They also come in many different concentrations with different values - so if the party is carrying 10,000 Sx, they're not carrying 10,000 different little crystals. In the Stineford magical library you can learn that this currency is generated naturally by monsters and that in very large amounts it can be used to ward against monsters by concentrating it. As Hilstara says:

Their power declines with time, as the Sx wear out once it has been refined. So as the group transport it, the crystals attract monsters, apparently.

So countries with huge amount of monsters are potentially rich and banks are (theoretically) not attacked by monsters. During your stay in Ardford, you will learn more about this process and how it really works.

ProN
Provisionary Notes are bill of exchange created by banks. ProN are used for Investments (mostly). The name is another of Sierra's little jokes.

Except plot driven events, for game balance you can't convert Sx to ProN or ProN to Sx.