ESXi automatically detects headless systems.
ESXi automatically redirects the DCUI over a serial port connection to improve headless detection. When ESXi automatically detects a headless system, ESXi will setup the serial port as COM1, 115200 baud, and redirects the DCUI over this serial port. The specific settings of com port and baud rate are read from the SPCR (Serial Port Console Redirection) table, if it exists. This behavior can be disabled using new boot parameters if the default settings are not acceptable. You can set the headless flag in the ACPI FADT table to mark a system as headless.