Panasonic AV-HS6000 2 M/E Live Switcher Main Frame & Control Panel (Dual Redundant Power Supplies) - Manual (Page 84)

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Chapter 5 Basic Operations — USK (Upstream key)
[Left] Masks the key left position.
[Top] Masks the key top position.
[Right] Masks the key right position.
[Bottom] Masks the key bottom position.