In plain language
Displays control brightness in different ways. Some dim the entire backlight together (global), some dim independent zones, and OLED panels control every pixel individually. The global dimming flag declares which behaviour applies.
Why it matters in your setup
Dolby Vision tone mapping uses that knowledge. A source that knows the display dims globally maps content differently from one that assumes per-zone control, because raising the level for a bright highlight also raises it everywhere else.
Practically, it is one of the settings worth trying when LLDV looks close but not right — particularly on projectors, where the assumptions built into a default data block often do not match the hardware.
What the DR HDMI 8K does with it
Global dimming is a switch in the DR HDMI 8K's Dolby Vision data block builder, alongside backlight control, backlight minimum luminance, YUV422 12-bit and 444/RGB 10 or 12-bit support. Build the block, send it, and the source uses those assumptions from then on.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.