StyleData

is a low-level representation of the contents of a style definition cell.

StyleData["style","environment"]

represents the contents of a style definition cell in the style environment "environment".

Details and Options

StyleData["style"] will inherit any definitions of "style" from parent stylesheets.
StyleData["style",StyleDefinitions->None] represents the contents of a style definition cell, ignoring any definitions of "style" from parent stylesheets.
StyleData["style",StyleDefinitions->StyleData["inheritedstyle"]] represents the contents of a style definition cell, inheriting any definitions of "inheritedstyle" from parent stylesheets.
Style definition cells are located in stylesheets and contain option values for a particular cell style.
Style definition cells have the form Cell[StyleData["style"],options].
Parent stylesheets are defined by a style cell with the form StyleData[StyleDefinitions->stylesheet], where stylesheet can take any filename specification allowed for the StyleDefinitions option at the notebook level.
Placement of styles in the Style menu is controlled by the MenuSortingValue option to the cell containing the StyleData expression.

Create a style definition for the cell style "MyTitle" and reformat the cell:

MyTitle

Define the cell style "MyText" for the "Presentation" style environment:

MyTextPresentation

Create a style definition for a cell that automatically numbers itself using a CounterBox and inherits style definitions from "Section":

SectionNumbered

Create a style definition for the "Working" environment:

Working

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