Searching the number under the image leads me to believe this is "Royalty-free stock photo ID: 1465044698
Novosibirsk, Russia – July 09, 2019: Citroen C4, close-up of the dashboard, speedometer and tachometer with orange backlight. modern car interior"
Took a look through a manual on manualslib but didn't see anything conclusive.
To be honest given that a similar bar seems to be present on the tacho in a region I would assert to be "high engine revs/warning/danger" I would presume this bar means "low fuel level when the gauge is in this zone". Being rendered thus also provides some symmetry to the layout and look of the tacho and fuel gauges. The only thing that I'm not totally convinced by is that this bar represents a 6th of the gauge and most other cars I've owned set aside more like an 8th of their gauge for the low fuel zone