What is the significance of the word "Door" in the novel, The Ten Thousand Doors of January?

In her parts of the novel, January capitalizes the word “Door” if it is a portal between two worlds. Regular doors are not capitalized. Consider January’s description of a closed Door: “a Door, now reduced to a mere door” (290). The capitals are used to categorize what sort of door to which January is referring. She notes that because her father is a scholar, he never capitalizes the word “door.”