x1 is an indentation/dent/impression/well/hole/bowl/pocket/invagination/sheath/divot in body x2.
This is in contrast with kevna, which may be a hollow interior space without connection to the unbounded exterior, and to torxesrtubnu, which requires full passage of the hole through the object. With the technology currently available to humans, all holes dug straight down into the earth/Earth are this sort of hole. For example: the well, which will hold liquid, in a coffee cup, rather than the region bounded by its handle and lateral surface, is an x1.