The Toshiba Thunderbolt 3 Dock is supporting DisplayPort Dual-Mode (DP++) standard via the Mini-DisplayPort on Video 2 output (since Mini-DisplayPort on Video 2 is supporting Multi-Stream Transport (MST) feature).
Above illustration is showing Video 1 (item 10) and Video 2 (item 11) group.
DisplayPort on Video 1 output is handling only normal DisplayPort signal and does not support DisplayPort Dual-Mode (DP++ standard).
DisplayPort Dual-Mode (DP++) standard allows DisplayPort sources to use passive adapters to connect to single-link DVI or HDMI displays. DP++ (Dual-mode) is an optional feature. Like DisplayPort on Video 1 group, not all DisplayPort sources support DVI/HDMI passive adapters.
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