On Mount Everest's north route, the one from Tibet, the uppermost camp ("Assault Camp") is at 27,300 ft (8,300 m) placing it in the so-called Death Zone which begins above 26,200 ft by convention and should be left within 24 hours as the human body slowly dies there. Is the camp just for rest (similar to the "Balcony" on the south route) or do people actually sleep there, in the Death Zone? If so, how long are climbers allowed to stay there and above, or how long should they?
To quote Wikipedia's article on the Death Zone:
In mountaineering, the death zone refers to altitudes above a certain point where the pressure of oxygen is insufficient to sustain human life for an extended time span. but we have supplemental oxygen, which is why most people don't die in the death zone.
I think the camp in question might be Camp VI from the 1933 British Expedition. There's an article in the Alpine Journal that suggests this camp was at 27,400 feet, matching the height in the question and that route used the North Col, where this camp is. So I initially wondered if the marker was historic, from before we knew as much as we do now about the effects of altitude.
But a camp in that area and at that altitude does appear to be used (search the page for "assault camp"), and was reached on 19th May 2016 before the summit the next day. I can't see anything in that article to support my conjecture in the comments that oxygen could be taken there for use at and above the camp, but certainly a stable location could be useful to cache anything not needed for the actual summit assault. Note that the expedition I linked continued the descent from the summit without stopping there again.
In fact 2 camps are shown on OpenTopoMap, on the North Col, with one at about 8200 m labelled camp 6, and the slightly higher one "Assault Camp", suggesting that they're not quite the same camp, but the same approach.
I've found a couple of results referring to sleeping with supplemental oxygen at or near this altitude. This one from 2007 (search "sleep on oxygen", uses a different route) refers to doing so at 7700m and stashing spare oxygen at 8300m. More closely relevant, this article about Mallory's final expedition says:
With plenty of oxygen available to them at C-6, any long delay would have caused them do the same as Finch did spend an extra night at C-6 and use oxygen for sleeping. [C-6 here is the camp I identified above]
which gives a couple of useful hints - it was possible even decades ago to get spare oxygen to camps at this altitude, and it's possible to use supplemental oxygen while sleeping. While the body's oxygen demands during sleep are less than while active, sleeping on oxygen with a decent flow could plausibly allow blood oxygen levels to rise a little compared to while active.
Returning to the definition of the Death Zone, supplemental oxygen would allow some time to be spent at the camp without the purely altitude/oxygen hazard that gives the zone its name