What is the Installer Passcode and why is it important?

The Installer Passcode is required during initial setup and must contain 4 to 12 characters including alphanumeric and/or symbol characters. This passcode prevents unauthorized changes to thermostat settings and is needed to access locked menus such as Advanced Configuration. Keep this passcode secure and share it only with authorized personnel.

From the official Honeywell Home Thermostat manual, page 40

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