To technically make undo/redo operations possible, completed drawing operations are stored sequentially in memory, or the stack. The level of support for drawing operations is divided into three categories based on how the operations affect the stack:

A command that changes the active model will clear the stack. For example, clicking File > Manage Session > Erase Current and accepting Yes will clear the stack. However, clicking File > Manage Session > Erase Current and then declining the model change with No will not clear the stack. The stack is not cleared during general navigational operations, such as toggling navigation panes, collapsing and expanding the browser, changing the drawing object filter, opening shortcut menus from the Drawing Tree or the Model Tree, unlocking view movement, or accessing context sensitive and online Help.