A reference mark is a nearby disk that is placed in order to help locate the primary station in later years, or to help reset it if if ever becomes damaged or destroyed. Most reference marks are not measured to the standard of the NGS and so do not have their own datasheets. However, some are, and do have their own datasheets.
Generally, a reference mark disk will say "REFERENCE" on its rim and there will also be an arrow across the center of the disk that points toward the primary station. The disk's designation stamping usually contains "RM" as well.