See: Tier 1 Capital. See: Tier 2 Capital.
You've got tier 1 capital...the good stuff. It consists of the high-level assets held by a bank, the kind of thing that can get cashed in during almost any emergency. It's key to judging a bank's solvency. Below that, you have tier 2. These assets don't have the same quality. They still have significant value, but in really dangerous economic times, the bank might not be able to rely on them to get out of a scrape.
Finally, you've got the tier 3 capital. It's unreliable. It consists of unsecured debt (meaning no collateral), subordinated debt (debt that gets paid off after some other senior lender gets their share), and other similar higher-risk holdings.
A bank doesn't want to have too much tier 3 capital, at least compared to the tier 1 stuff. In fact, regulators insist that a bank carry no more than two-and-a-half times the amount of tier 3 capital as it has tier 1.
