Global Currency: Played straight, twice, and arguably more often.
The first of them is the Empire’s esteyn, acceptable everywhere in it and existing in the interests of internal free trade. It’s a quasi-fiat currency, managed by the Imperial Board of Money and Values to avoid either inflationary or deflationary tendencies to provide a reliable store of exchange-value. It’s acceptable anywhere the Imperial writ runs, in quanta from the micro-esteyn to the mega-esteyn, and is denominated in a truly remarkable range of values as well as its official ones, inasmuch as to simplify the transition, most of the local currencies which formerly existed were redefined as new denominations of the esteyn.
(The management is done on the basis of an energy/cycle index, these days, modified for secular productivity changes due to innovation. It used to be done on the basis of a rather broader basket of commodities, likewise modified, but the advances in nanofacturing and automation technology have thinned it quite considerably, in consequence. Also, while technically it’s done by the IBMV, in practice, in the modern era, most of the day-to-day work is done by a massively-distributed AI which extends itself all through everywhere there is an economy and keeps track of those statistics which it needs to know to make things work.)
The second is the Accord exval, an interstellar exchange currency between and supported by the signatories to the Accord on Trade, and managed by the Conclave and the Galactic Trade Association on their behalf – or rather by their tame AI – to be valued at the real-time floating average value of the currencies of each of the members. (This makes it rather more volatile than the esteyn, both because many members prefer to hide or play silly-bugger games with their actual economic numbers, and because enough of its members are AI-phobic enough that the GTA’s AI is nowhere near as intelligent or self-directing as the Empire’s Fiscal Prime. Some people evidently find it hard to turn control over to a machine that’s smarter than they are.) The exval is not, for the most part, a real currency – it’s money of account and exchange currency, used to optimize cross-polity electronic currency transfers and make it easier to track the real values of foreign-held assets – and to let working spacers and tourists know what the actual prices of things are as they spend (most polities that receive any amount of tourism, and just about all the starports/startowns, dual-label goods in local money and Accord exvals; which one gets the round number and which one floats tends to vary by location).
There are a number of other currencies of general acceptability; the venerable gAu, gAg, and gPt can be traded anywhere, although you need so damn much of them given modern mining technology and its effect on the value of the underlying metals, it’s hardly worth bothering anywhere off Hicksworld. The energy (usually antimatter) backed, fully convertible, ergcred is sometimes seen, especially out towards the Rim Free Zone. A few of the higher-infotech societies out there occasionally talk up the virtues of the finality – and use it between themselves – backed by irreversible computational operations, but have trouble selling it to people of less infogeekery.