The Dimension Tool is responsible for creating Dimension Entities.
Find one of the Dimension Line Tool icons 
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on the Entity Toolbar (color coded blue) and click on the tool. The tool is now active and ready for use. This set of tools is part of a Flyout toolbar.
1.
Aligned Dimension Tool (default): A linear dimension for objects with unusual shapes, or with edges at unusual angles; the dimension line will always parallel the imaginary line connecting the origins of the two extension lines.
2.
Linear Dimension Tool: A linear dimension that will either be oriented along either the X-axis or the Y-axis (your choice).
3.
Ordinate Dimension Tool: A dimension of an X-axis or Y-axis point.
4.
Radial Dimension Tool: A dimension for the radius of a circle.
5.
Diametric Dimension Tool: A dimension for the diameter of a circle.
6.
Angular Dimension Tool: A dimension for an angle (in radians or degrees).
Also See:
Dimension Entity
Manage Dimension Styles
Flyout Toolbar