What is the difference between SF-2 and SFF-2 silicone wires?

Both are standard Appliance Wiring Materials (AWM) made of silicone and fiberglass, but they differ in mechanical flexibility (the extra “F” stands for Flexible):

SF-2: Utilizes a solid or standard stranded conductor (Class B or C). Best for fixed internal wiring.

SFF-2: Utilizes a highly flexible stranded conductor (Class H or K). It is ideal for tight routing in small lighting fixtures or appliances subject to vibration.