F2F means "forwarded to Firewall", a.k.a "Slow Path". It applies to any packet that cannot or should not be accelerated.
The term is in fact mentioned in multiple guides and SecureKnowledge articles, for example, in sk153832, quoting:
"Firewall path / Slow path (F2F) - Packet flow when the SecureXL device is unable to process the packet (refer to sk32578 - SecureXL Mechanism). The packet is passed on to the CoreXL layer and then to one of the Core FW instances for full processing. This path also processes all packets when SecureXL is disabled."
Exactly the same statement is used in sk98722.