The PPSA establishes a priority hierarchy for different types of security interests. There are several types of security interests that can attach and be perfected second yet defeat security interests perfected first, regardless of knowledge of the first-perfected security interest. These are predominantly PMSIs, and security interests perfected by control over principally financial assets (shares, bonds, ADI accounts – see paragraph 17.6.2 of Chapter 17 (Perfection) for a list of collateral that can be perfected against by control).
PMSIs were well known to the common law. The PPSA codifies a (much broader) version of the common law PMSI principle. Further, leases and conditional sales by retention of title transactions were not thought of as PMSIs before the PPSA, but perform a similar role, and accordingly, are rolled into the PMSI principle under the PPSA. See Chapter 18 (Priority) for discussion.