On your inflight start enveloppe ,from Fl240 to FL150 the speed of 281 kts is the minimum to be in the windmill area.
Below FL 150 you need a minimum of 300 kts to be within the in flight start enveloppe for windmilling.
The 281 kts is written on the chart and it is for the NG.
Maybe that speed represents the speed at which a successfull relight was demonstrated,again it says successfull starts may be obtained outside of the enveloppe but not assured
like any other would be...