StyleData will inherit any definitions of from parent stylesheets.
StyleData["style", StyleDefinitions->None] represents the contents of a style definition cell, ignoring any definitions of from parent stylesheets.
StyleData["style", StyleDefinitions->StyleData["inheritedstyle"]] represents the contents of a style definition cell, inheriting any definitions of from parent stylesheets.
Style definition cells are located in stylesheets and contain option values for a particular cell style.