Properties are either inheritable or not--inheritance occurs within the result tree
Inheritable properties may not have any effect on a particular formatting object on which they are set but their values may affect descendant objects
Properties have some initial (default) value
Therefore, there is always a value defined for every property on every formatting object (either inherited, defaulted, or assigned), though not every property will be relevant to every formatting object
The description of each FO says which properties are relevant to it and how