One thing that might be difficult to separate conceptually when working with financial API’s like the BFS API is the difference between cash and currencies.
Cash is an asset class just like stocks, bonds etc. while currencies are the object that describes which type the cash is. Just like a stock can be a common stock or preference share.
An account can keep assets like stocks, bonds or cash but it cannot keep a currency. A currency is something that relates to a cash asset.
In the BFS API we have the asset entities grouped together here and the currencies here.
Another concept that is good to grasp is how an asset can be valued. Typically we value an asset in relation to a currency but we could just as easily value an asset like a stock relative to another asset like gold.