Difference between "Level" and "Flow" in the Z.1 Fed data The Z.1 Fed data can be found in FRED.

I notices that many similar data sets are labelled "Flow" or "Level". For example,


Financial business; total liabilities, Flow
Federal government; total liabilities, Level
Financial business; total financial assets, Level


And sometimes, the Level version for a Flow series doesn't exist or vice verse.

What is the difference?
Say you have a balance sheet that measures a country's total stock of financial assets and liabilities for a given year. If you have a bunch of these balance sheets for several years, those data points are the actual levels of total assets/liabilities. 

Now suppose you decide to subtract the levels that correspond to year $t-1$ from the levels that correspond to year $t$. That's what the Fed is doing with these data series -- the flows are the first-difference of the levels.