Level 1 (red letters) represents the lowest security and is easily picked or bypassed with minimal skills. They do not contain security pins or any security features. Level 1 Examples: Master Lock #3 and TSA locks.
Level 2 (orange letters) is slightly better than level 1 in terms of resistance to picking and bypassing. It may contain security pins or security features and requires an intermediate level of picking experience to defeat. It is a good, general purpose lock. Examples: American brand padlocks containing security pins and a anti-bypass wafers, Master Pro-series locks containing security pins.
Level 3 (yellow letters) locks are resistant to picking and cannot be bypassed. They contain security pins, usually a mix of types and require an advanced level of experience to defeat. Alternately, Level 3 locks may contain standard pins if manufactured to very tight tolerances causing multiple pins to bind simultaneously. Level 3 locks are excellent for high security (attended) applications. Example: Assa padlocks and cylinders, Best SFIC (and others), Sargent cylinders.
Level 4 (light green letters) locks are highly resistant to picking and cannot be bypassed. They contain a mix of security pins, anti-picking materials, paracentric keyways, and are manufactured to high tolerances. Level 4 locks require an advanced level of experience to defeat and require considerable time to do so. Level 4 locks are excellent for high security (unattended) applications. Examples: Ruko D12 cylinder, Medeco 5-pin locks, and Mul-t-lock Interactive series.
Level 5 (dark green letters) locks cannot be defeated in a reasonable amount of time by any means. They contain a wide variety of security features (armor, extraordinary pinning or locking design, special metals, and tightly restricted paracentric keyways. Level 5 locks are highly resistant to physical attacks. They require special tools or training to decode or defeat, which also requires considerable time. Examples: Abloy Protec2, Mul-t-lock MT-5+, Medeco 6-pin locks, and the Assa Desmo.