Using the Anchor with EQ dimensions

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In a multi-segmented, equality constrained dimension, you can use the anchor symbol to designate the anchored element. The element that is anchored remains stationary while you move other elements in a dimension.  

• Click and hold the anchor symbol. A witness line appears for the anchor.
• Drag the anchor symbol to the element you want to anchor.
• Move any of the unanchored elements referenced in the dimension.
• The element accompanied by the anchor does not move.

From:
Anchoring Elements Referenced in Multi-Segmented Dimensions - WikiHelp

Also refer to my previous post:
What does the Revit equality dimension anchor do?