1. When the aircraft has cleared the runway.........................
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1. You're out of the way and another aircraft can physically fit onto the runway.
As has been said as long as the aircraft has vacated the physical runway itself then next aircraft movement can take place.
I beg to differ.
About 70% of our traffic are Heavies and therefore have wingspans exceeding our 60 metre runway width. Having seen several arrivals prop on the rapid exit short of the stop bar for no particular reason, it would be a brave person to fire the next departure just because the tail was past the runway edge.