Mass Effect & Mass Effect 2
The standard credit was established by the Citadel's Unified Banking Act as the currency of interstellar trade. The credit has a managed floating exchange rate, calculated in real time by the central bank to maintain the average value of all participating currencies. Some regional currencies are worth more than a credit, some less.
Hard currency can be stolen or counterfeited, so electronic fund transfers are the norm. More importantly, physical transactions cannot be easily tracked, making them ideal for tax evasion or the purchase of illegal goods.
When the Alliance joined the Citadel, its various national treasuries were linked into the credit network. A human with a bank account of Mexican pesos, Japanese yen, or Indian rupees can purchase any item priced in credits at fair market value. All economies that participate in the credit network are required to price items in both local currency and credits.