Through which communication channel does an employee formally convey feedback, complaints, or suggestions to higher-level management?

Upward communication flows from subordinates to superiors in the organisational hierarchy examples include feedback, grievances, suggestions, performance reports, and status updates sent to managers or senior leadership. Downward communication flows top-to-bottom (instructions, policies). Horizontal communication occurs between peers at the same level. Diagonal communication cuts across departments and levels. Grapevine is informal communication outside official channels. The question specifically asks about formal channels from employees to upper management that is upward communication.