The three types of fall protection systems include: 

• Fall Prevention 
• Fall Restraint 
• Fall Arrest

OSHA's key fall protection systems are categorized as: 

• Guardrail Systems 
• Safety Net Systems 
• Personal Fall Arrest Systems 
• Positioning Device Systems

The difference between a Self-Retracting Lifeline (SRL) and a Personal Fall Limiter (PFL) is that a PFL is a type of SRL that is worn on a harness and designed to be small and lightweight. Meanwhile, an SRL is heavier and typically attached to an anchor point. Both SRLs and PFLs are used to arrest a fall, quickly locking the lifeline when the line extends rapidly, like a car seatbelt. 

The most secure fall protection system is the one that can stop a fall altogether. Prevention systems, such as guardrails, typically provide the most reliable protection when workers remain inside a guarded safe zone. However, guardrail systems may not work with certain jobs and it's important to choose a system based on the needs of a specific project or applications.