Numbers 19:16

In an open field. The Rabbis extrapolate, this includes the “gollel” and “dofek.” Its plain meaning is that in the open field, where there is no tent, the corpse conveys defilement through touching.

And whoever shall touch not one who hath died in his mother's womb, but who hath been slain with the sword on the face of the field, or the sword with which he was slain, or the dead man himself, or a bone of his, or the hair, or the bone of a living man which hath been separated from him, or a grave, or a shroud, or the bier, shall be unclean seven days.

AND WHOSOEVER TOUCHETH IN THE OPEN FIELD. “Our Rabbis have said that this includes the upper board and the side board [of a coffin, when found in the open field]. And the plain meaning [of the verse] is: in the open field where there are no tents, a corpse conveys uncleanness [only] by contact.” This is Rashi’s language. But in the opinion of the Sages the [law that the] upper and side boards of the coffin [convey uncleanness] is not derived from Scripture, but is a tradition [handed down by Moses who received it at Sinai]. Therefore a Nazirite [who has become impure by coming into contact with them] is not subject to [the law of] shaving his hair, and likewise [one who becomes impure through contact with them] is not liable to punishment for defiling the Sanctuary [by entering it when impure] or [by eating] the hallowed offerings [in such a state of uncleanness]. Rather, [the expression] in the open field is to be understood according to our Rabbis in its literal sense, i.e., that he touches a person who is slain by the sword, who has fallen in the open field, and there is nothing covering over the corpse. And since this verse is now [seemingly] superfluous, [since it has already been stated in Verse 11 above that one who touches a corpse becomes impure], the Rabbis derived from it the following interpretation, and they said in Tractate Nazir in the Chapter “The High Priest:” “And whosoever toucheth in the open field. This refers to one who ‘covers over’ a corpse” [and tells us that he becomes impure], since by covering over it he “touches” the open field. [According to this interpretation of the Rabbis the expression] bachalal cherev (slain by the sword) is lacking the [connective letter] vav — u’bachalal cherev, [meaning: “or slain by the sword”]., i.e., who leans over a corpse or covers it with his body, because in doing so he “touches” the corpse; 2) or whosoever touches directly one slain by the sword. And since Scripture says ‘pnei’ hasadeh [literally: “the face” of the field] and does not [just] say “the field”, Rabbi Yishmael further interpreted it to exclude the [case of a] foetus which died whilst still in its mother’s womb [so that if the midwife put her hand inside the mother and touched it, she does not become impure], because it is impurity which is “swallowed up,” and is not “upon the face” of the place.
Scripture states slain ‘by the sword’ because …

וכל אשר יגע על פני השדה, if one enters a closed area, roofed area in which the remains of the deceased are present one becomes ritually unclean, whereas if the remains of the deceased are in an open area such as a field, only direct contact with the deceased confers such ritual impurity.