Lots of great points being made!
From all of the canon that I've seen, the simple explanation is that there is no SINGLE system.
In financial terms, the time delay inherent in interstellar travel creates RISK. The risk is dealt with in different ways for different entities.
A planetary government or megacorporation or noble will have a credit history that is known to individuals and corporations within his sphere of influence. Those entities will, as a matter of course, extend credit for a fee. If, for example, the Duke of Regina wanted to borrow money while visiting Darrian, he would either have:
a. Transferred assets - credits (real or electronic, direct (his own) or indirect (draft from some financial entity)), or
b. Real items like goods, metals, valuable luxury items or whatever.
Those assets would either be sent or arranged ahead of the visit, travel with him, or be settled afterward based on the RISK inherent in dealing financially with the Duke. The key is that it would have been whatever is appropriate for the destination(s). That could be a case of whisky on a TL4 world, a portable fusion generator on a TL9 world, or an electronic wallet with active credit/debit on a TL12 world.
This sphere of influence could be local, regional, planetary, sub-sector, sector, domain or state-wide.
Once you are outside your personal sphere of influence, you no longer have an available credit history. You are therefore a greater risk. You must then travel with something of value like actual assets, or the credit of some entity that does have influence in the area you are in. Relying on someone else's influence is the key. It's the way modern banking works. I don't know the guy from another city that I sell something to, but if he has a TAS membership or an account with an megacorp, a bearer bond, or a letter of credit from the Duke of Regina, I do know those entities. They all have credit histories that reduce risk.
Now let's scale it down to a free trader, or a party of travellers. After alot of hard work, a merchant will have developed a reputation on the worlds which he visits. PLEASE NOTE: this can be good or bad. If you tick off the financial authorities on a world, the increases your risk, possibly to the point where they won't do business with you.
Where you have a reputation, you will probably be able to borrow, for a fee, against your Credit Rating, or trade real goods at preferred rates. If you're on a new world, you either:
a. are dealing with a local institution or individual who has access to your reputation/credit rating (you are therefore operating within your sphere of influence),
b. are travelling with the local equivalent of a financial instrument, bearer bond or letter of credit that the world you are visiting recognizes, i.e. you are making use of someone else's sphere of influence). This is the premise behind the Imperial Credit, or
c. are carrying goods that have value on that world.
What would traveller's have?
Visiting Low Tech - Here the key is physical form. Some examples are precious stones or metals, local currency (acquired off-world), art objects, masterwork items for that tech level, attractive trading goods, a bearer bond or letter (actual) of credit from an authority recognized by the world. Note that some low tech worlds will be sophisticated enough to accept Imperial Credits if they take physical form (like a letter of credit),
Visiting Mid and High Tech - as above, as well as scientific and tech items sustainable on that world, and intangible assets (knowledge and electronic funds).
Paul Nemeth
AA