Center lines and object centers are used to properly reference dimension lines on CAD drawings.

Center line
The Center line tool works much like the CAD Line tool. Use it to bisect a symmetrical object or show exploded view lines. Click and drag to position the line, then release the mouse to finish creating the line. Hold down the shift key while dragging to constrain the Center line to 45 degree increments.

Object center
The object center tool determines the center of an object based on its internal bounding box. Create any path or object and select the Object center tool. Click on the artwork to create its object center lines.