Final answer:
The subsystem that handles software support and distribution for Standard Terminal Automation Replacement System Operational Sites is the Software Distribution Unit (SDU).
Step-by-step explanation:
The Standard Terminal Automation Replacement System Operational Site (SOS) subsystem that provides software support for the site and site-to-site data communication for software distribution is the Software Distribution Unit (SDU). The SDU is responsible for distributing software updates, patches, and new modules to the various components of the SOS. It ensures that all machines within a site, as well as multiple sites, are running on the same software versions, providing consistency and the latest improvements across the system. It's an essential part of the infrastructure for maintaining the operational integrity of the air traffic control system.