John Gill Commentary Numbers 19:16

"And whosoever in the open field toucheth one that is slain with a sword, or a dead body, or a bone of a man, or a grave, shall be unclean seven days." — Numbers 19:16 (ASV)

And whosoever toucheth one that is slain with a sword in the
open fields
That is killed by another, that dies a violent death, either by the sword or other means; one that touched such an one was unclean, or that touched the sword with which he was slain, as the Targum of Jonathan adds: "or a dead body": that dies a natural death, or suddenly, or in any way:

or a bone off a man ;
dug out of a grave, and lying by itself:

or a grave ;
the Targum adds, either the covering or side of a grave:

shall be unclean seven days ;
all which has respect to the defiling nature of sin, which is the cause of death and the grave.