Sony IPELA PCS-XL55 Operating Instructions Manual page 38

You can also press the PC/RGB button and VC button to switch between
videoconference display, two-picture display with the videoconference picture
as the main picture, and two-picture display with the computer picture as the
main picture.
For details on using the system as a computer display, see "Chapter 7:
Computer Display Function".
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Turning the System On/Off