Currency

There are two types of money in the game and they can't be used interchangeably. Both are named with puns: Sx and ProN. The amount you currently posses is displayed in the lower left corner of the menu screen. (The ProN counter appears only after a certain point in the game.)

Sx
Sx is the day-to-day currency of the game. You use it mainly to buy stuff and services.

Physically, Sx are crystals that 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 Sx are generated naturally by monsters (which explains why monsters drop Sx when killed) and that in very large amounts Sx can be used to ward against monsters. The latter point is relevant to an optional quest at a certain point of the game.

ProN
"ProN" is short for "Promissory Notes" - bills of exchange created by the banks and used for large-scale financial transactions, such as business Investments and hiring mercenaries.

For game balance, you can't convert your Sx to ProN, and you can convert your ProN to freely usable Sx only at certain points of the plot. If it's possible, conversion can be done by talking to Megail or at a bank. The rate is (usually?) 1 ProN = 1 Sx, but you can only convert fixed amounts of several thousands at once.