The Link Proportions button is found within the Items tab.
The purpose of this button is to activate or deactivate the automatic proportional dimensioning of any given item.
When it is active, it appears highlighted within the ribbon.
Below are some example images to give an illustration of the difference when the Link Proportions button is active.
To keep it simple, the dimension for Length will be altered from 1010mm to 500mm. Notice the values of the Depth and Height dimensions for each example.
Default dimensions
Dimensions with Link Proportions active
The Depth and Height adjusted themselves to be in proportion to the new Length.
Dimensions with Link Proportions inactive
The Depth and Height remained exactly the same as they were prior to adjusting the Length.
If required to revert back any changes made to the adjusted item, simply click on the Reset Graphic Item Dimensions button just below the Link Proportions button. This will reset the dimensions back to their original values.