Red represents the people.
Red pins on the map denote just how much directable manpower is available to a holding. Often, outposts wont have any- and most settlements will have one. Red is usually expended to trigger special abilities of a holding although its main function is to facilitate the production of new settlements, or other manpower intensive projects.
Since red represents an effect of workingpopulation, raising certain military units, “levy” a red pin from the holding, meaning that until disbanded, that red is “tied up” in the unit.