The "R[eserve]" idea goes back (at least) to early Volkswagens, which had no fuel gauge at all. There was a fuel valve (under the dash) similar to the ones motorcycles have with an "on" and a "reserve" position (maybe even an "off," I've never actually seen one). When the engine started to sputter from lack of fuel, you turned the valve to Reserve and went hunting for a station. Over time, you learned how many miles you would get before you "went on reserve" and adjusted your driving accordingly.

The vehicles with gas gauges are set up in a similar fashion. When the gauge gets to "R," you have a certain amount of fuel remaining. After some additional time (miles) have elapsed, the warning light comes on and then you'd really better be looking.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970