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.

Anchoring Elements Referenced in Multi-Segmented Dimensions – WikiHelp

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