Sheet format defines the sheet size and elements on the sheet. The default sheet format is A3 with a border, title block, and three extensions for more information. The Sheet Format dialog box provides you with the ability to change sheet size and add, change, or remove elements on the sheet.
When you add a new sheet, it appears with the same format as the active sheet. Each sheet in a drawing can have different formats.
Extensions are additional title blocks that contain more information about the drawing or specific parts in the drawing.