This file name refers to a Cisco IOS Software Release 15.5(1)SY10 specifically designed for the Catalyst 6800 Supervisor Engine 6T (Sup6T)
Full MPLS & VPLS:
Essential for Service Providers or massive enterprises requiring Layer 2/Layer 3 VPNs across their own infrastructure.
s6t64 — The Hardware Target
To the uninitiated, this looks like a cat walked across a keyboard. To a Network Engineer, it is a precise manifest of hardware, features, and security.