The U.S. National Park Service defines a Historic District as:
"a geographically definable area, urban or rural, possessing a significant concentration, linkage, or continuity of sites, buildings, structures, or objects united by past events or aesthetically by plan or physical development. A district may also comprise individual elements separated geographically but linked by association or history."
When an NRHP nomination for a historic district is submitted to the National Park Service, the narrative separates the structures within the district into 'contributing' and 'non-contributing' structures. 'Contributing' structures are buildings, sites, structures or objects adding to the historic significance of a district.
To waymark a structure, it must be designated as a 'Contributing structure or building' to a historic district listed on the National Register of Historic Places. Waymarks should give a sense of the Historic District they are contributing to. Ideally, your contributing building should be specifically mentioned in the NRHP narrative. Many states have their NRHP narratives accessible via the internet. Some internet resources are posted in the category forums.
Please do not submit contributing buildings that have an individual listing on the U.S. National Register of Historic Places (NRHP). These buildings can be listed in the NRHP category.