KiCad supports nesting schematics hierarchically to simplify the creation of complex schematics designs.

A hierarchical schematic uses hierarchical sheets (SCH_SHEET objects) to reference a given schematic file (SCH_SCREEN objects). Each SCH_SHEET corresponds to a schematic file handled by a SCH_SCREEN object. A SCH_SCREEN object contains schematic drawings and has a filename to read/write its data.

In simple hierarchies one SCH_SHEET object is linked to one SCH_SCREEN object.

In complex hierarchies the a SCH_SCREEN object shared by more than one SCH_SHEET object. Therefore all sub-sheets can also be shared. So the same SCH_SCREEN must handle different symbol references and unit selections depending on which sheet is currently selected, and how a given subsheet is selected. SCH_SHEET objects share the same SCH_SCREEN object if they have the same schematic file.

In KiCad each SCH_SYMBOL and SCH_SHEET receives a UUID when created. These UUIDs are chained together to form SCH_SHEET_PATH objects that allow access of instance data in the hierarchy. The sheet paths have the form /ROOT_SHEET_UUID/SHEET_UUID/SUB_SHEET_UUID/...

For a given SCH_SCREEN SCH_SHEET_PATH objects must: 1) Handle all SCH_SYMBOL references and unit instance data. 2) Handle all SCH_SHEET page number instance data. 2) Update the currently displayed sheet SCH_SYMBOL references and SCH_SHEET page numbers.

The class SCH_SHEET_PATH handles paths used to access a sheet. The class SCH_SHEET_LIST allows one to handle the full (or partial) list of sheets and their paths in a complex hierarchy. The class SCH_SCREENS allows one to handle a list of SCH_SCREEN objects. It is useful to clear or save data, but is not suitable to handle the full complex hierarchy possibilities (usable in flat and simple hierarchies).