The legal term in rem means "against or about a thing." It refers to a lawsuit directed towards property, such as land, rather than against a person. As such it lends itself to some unusual case stylings, such as the actual case United States v. Various Pieces of Semiconductor Manufacturing Equipment, 649 F.2d 606 (8th Cir., 1981) (the company that was caught trying to sell them to the Soviets claims the long delay between seizure and forfeiture proceedings was a due process violation).
If the dispute is over title to property is the issue, then action is considered "in rem." The location of the property then determines which court has jurisdiction.
Contrast "in rem" with "in personam."